Intercontinuity Crossover



"Jigsaw: You wallow in hypocrisy, Angus. Your life has been dedicated to nonviolence, which you advance by jury-rigged poisons and explosives, pretending that using one form of violence over another makes you better than others... Well, now it is time to see how far you're -- did you just turn your manacles into laser beams? MacGyver: Yep. Jigsaw: ...Crap."

- Hypothetical MacGyver-Saw crossover.

Having a Crossover between two related shows is one thing—hey, it's not inconceivable that some of the Cheers cast should turn up in its Spin-Off show Frasier, is it? Or that the CSI team might help out the guys in CSI: NY every now and again.

But what happens when Jay and Silent Bob turn up in Scream 3? Or Cliff and Norm from Cheers book into St. Elsewhere for medical attention? Or a police officer from the grittily realistic Homicide: Life on the Street arrests Fox Mulder of The X-Files? Or the CSI team finds a corpse in a hotel that isn't a corpse, just Angel? Well, then you've got yourself a different kettle of fish altogether. And to add insult to injury, the fish are probably from Finding Nemo and The Little Mermaid.

There are a few reasons why this happens. Sometimes it's because both series are owned by the same network or company, and it wants to improve the ratings on the one show by having the cast from the more popular show appear in it. Sometimes it's because both shows share writers or producers and they think it would be cute to link the two. And sometimes it's just because someone somewhere thought it would be cool.

Most crossovers of this kind are between properties that are owned by the same company or person, which makes legal wrangling easier. But sometimes characters will cross over into another company's properties, like the occasional events from rival comic book firms DC Comics and Marvel Comics. And, of course, anything goes when it comes to Fanfic.

It makes quite a difference whether the crossover reveals that the characters have supposedly been sharing the same world the whole time; or if they're "just" from Another Dimension. While not as big a deal if both series are "relatively" realistic, such as Crime Dramas, if each show has its own version of fantastic events a la Fantasy or Science Fiction; then the implication is that these elements have been existing side by side the whole time, unaware of each other. It also makes for some great Fridge Logic. "Hey, Problem A for Cast B is no big deal, if Cast A and B are friends, why don't they just ask for help?" Of course, Reed Richards Is Useless.

For more information and examples, see Thom Holbrook's Crossovers and Spin Offs Master List, which documents all known crossovers between TV series, except for cartoons. With this information, the site has been able to identify no fewer than 38 contiguous TV "universes" (as of March 2008) which are home to widely disparate (and apparently unlikely) combinations of series from different eras and networks. For instance, see Holbrook's "Group 2" -- ninety-four series (as of March 2012) ranging from Six Feet Under and Everybody Loves Raymond to Law and Order and The X-Files, taking in Mash, Cop Rock, and Home Improvement along the way. Thanks to the baroque web of interconnections and crossovers, it is suggested that these shows all inhabit the same universe. (Holbrook also discovered the "Jean-Claude Van Damme is dead" plague that poses a grave threat to any future crossovers including Star Trek and other Group 10 shows.)

It has been noted on this and other sites that the web of crossovers connects many shows, directly or indirectly, to St. Elsewhere. Since that entire series was shown in its final episode to be the fantasy of an autistic mental patient, we might reasonably conclude that the whole of television is All Just a Dream. In the words of Dwayne McDuffie, one of the executive producers of Justice League, "The last five minutes of St. Elsewhere is the only television show, ever. Everything else is a daydream."

For the Web Comic equivalent of the TV crossover list, see the Webcomic Crossover & Cameo Archive.

For a fanfiction equivalent, see this image. (The map needs to add City of Villains, but nobody's complaining really.)

When this trope happens with three or more series instead of two, it's a Massive Multiplayer Crossover. When the two settings are blended so as to be one unified setting that partakes of elements from both, that's a Fusion Fic.

When each others' works of fiction appear in-universe as fiction in a stable fictional loop, that's Mutually Fictional. When a work of fiction itself appears in its own story, that's Recursive Canon.

Compare Crossover, Fake Crossover. See also Vegas Crossover, Folgers Crossover, Transplant, Fully-Absorbed Finale, Guest Fighter and Ultimate Showdown of Ultimate Destiny.

Anime & Manga

 * Part of the Red Ribbon Army story arc on Dragon Ball had Goku chasing Captain Blue to Penguin Village, the setting of Akira Toriyama's earlier manga/anime, Doctor Slump, and featured appearances by Arale, Dr. Norimaki, and several other characters. This appeared in the manga and anime (since both shows were made by Toei and aired on Fuji TV); moreover, when Doctor Slump was animated again in 1997, a new Dragon Ball crossover was written in.
 * Toriyama already did that in Dr. Slump itself: there was one chapter where the characters traveled to Wonder Island, the set of the titular manga (and Toriyama's very first published work).
 * The AIR/Kanon crossover manga. (Key Visual Arts)
 * Apparently mostly because the author of Fairy Tail really liked Flunk Punk Rumble they made a crossover called Fairy Megane.
 * That the author of Flunk Punk Rumble's favorite author happens to be the author of Fairy Tail probably helped, as well a former assistant of his. What's most stunning of all is that despite being SERIOUSLY WTF, it actually ain't half bad.
 * The Fate Stay Night X Magical Girl Lyrical Nanoha and Magical Girl Lyrical Nanoha X Fate Kaleid Liner Prisma Illya collaboration manga that both started in the March 2010 issue of Comp Ace magazine.
 * And then there's the Detective Conan/Lupin III crossover TV special...
 * This was because the anime adaptations of both series were produced by TMS Entertainment.
 * Dragon Ball and One Piece in Cross Epoch.
 * At one of the Jump Fiesta events they featured another crossover of Luffy and Goku going up against Eneru above the city of Tokyo. Other characters get short cameo and even Ryotsu of Kochikame made an appearance (as seen here). They would later do this again only this time against Arlong and Astro Boy was throw into the mix.
 * Slayers VS Orphen
 * Rodimus's backstory in Transformers Energon states that he left Cybertron with a band of Autobots and "traveled into the future", which reflects the ending of Transformers: The Headmasters, in which he ventures off into space to find/found a new home planet.
 * To promote the Jump Super Stars game for the DS, a manga was included for those who preordered drawn by the people behind Eyeshield 21. A standard "sucked into a world to defeat the villain" affair but damn if its not cool seeing Naruto, Luffy, Goku, Toujou, and Bobobo work with Seta to kick Dr. Mashrito's butt. Check it out here.
 * It's not much, but in G Gundam, during the finale episode where all the world's Gundams had banded together to fight the Dark/Devil Gundam, you can see Gundams from other series in the mob. For example, both the standard RX-78-GP01 and the RX-78-GP01 Full Vernian, the Wing Gundam, RX-78-2, etc. They weren't kidding when the said they had all the Gundams!

Comic Books

 * The Doctor showed up in one of the Buffy Season 8 comics for a panel (along with Rose). Suddenly London's forgetfulness makes sense.
 * And now there's this. It's... beautiful.
 * Beginning with Predator vs. Aliens, the two extraterrestrials have done battle (either one-on-one or multiplayer deathmatch) with essentially every superhero ever created, no matter how illogical the match-up. Taken to ridiculous extremes in Superman and Batman vs. Aliens and Predator.
 * Perhaps the most famous of these stories was Wild CATS/Aliens which actually had an impact on the former's continuity and saw the deaths of several members of Stormwatch to allow Warren Ellis to clear the decks to create The Authority
 * Despite their rivalry, DC Comics and Marvel Comics have had numerous crossovers between their two companies, leading to meetings between Superman and The Mighty Thor and in particular Batman and Captain America (comics), the two companies main Badass Normal heroes, and also confrontations between heroes and villains from each, such as Spidey taking on the Joker. The whole thing even has a standing Hand Wave established in the late 1990s, that the Marvel and DC universes will occasionally start to merge on their own, leaving teams from the two inhabiting the same continuity all of a sudden and remembering each other as if it's always been that way. For instance, in the X-Men/Comicbook/TeenTitans crossover, at one point they even discuss bringing in the Justice League and Avengers to help.
 * An interesting example: X-Men and Teen Titans. Starfire learns languages by kissing people, and Nightcrawler thinks that's terrific.
 * An unfortunate side-effect of this is that the closest Jubilee has ever come to a serious relationship is with a guy who lives in another universe.
 * Some comics by Roy Thomas saw a team-up between Robert E. Howard's Conan the Barbarian and Michael Moorcock's Elric of Melnibone.
 * On Elric's side this makes sense, as his home continuity is explicitly one of many parallel worlds.
 * Judge Dredd has teamed up with Batman on at least three occasions; given Bats' vigilante status and Dredd's fascist asshole status, there was some distinctly Teeth-Clenched Teamwork on both sides. Dredd has taken on Aliens and the Predator as well.
 * And in Two Thousand AD publications themselves, Dredd has crossed paths with Johnny Alpha from Strontium Dog twice (Note that both of these strips have been created by the same people) and has seen a whole score of cameos from outside characters during the Helter Skelter story arc.
 * One of the single most bizarre Intercontinuity Crossover, ever: Archie Comics Meets The Punisher
 * Superdickery.com has some that are even worse.
 * Superman/He-Man. It didn't suck as much as you thought it would, chiefly because this was before the cartoon debuted, and He-Man behaved more like the loinclothed barbarian he is. Also, no Orko.
 * The weird thing is that the Archie/Punisher crossover didn't suck. And there's not a single out-of character moment. The Archie EIC at the time admitted the comic came about from him meeting some Marvel guys at a cafe and talking.
 * Dark Horse Comics gave us Tarzan versus Predator At The Earth's Core.
 * Illyria of the Buffy Verse has appeared in Peter David's Fallen Angel. Note that David also wrote an earlier Illyria-one-shot.
 * Top Cow had an extended crossover with Marvel where the companies' characters live in different universes and which is probably in continuity, although several crossovers are pointedly not (like the one where Wolverine gets married to Sara Pezzini), and a Darkness/Batman crossover which featured an important in-continuity event that affected the Darkness series itself, even though it couldn't possibly be in continuity.
 * Captain Atom: Armageddon thrusts the DC Comics hero into Jim Lee's Wildstorm universe. The mini-series functions as a Cosmic Retcon for Wildstorm, and allows those comic readers who generally hated the ridiculousness of The Authority watch them get their heads handed to them.
 * Mars Attacks! the Image Universe! Notable in that The Savage Dragon actually kept it in-continuity, with an issue that gave a basic overview for those who didn't read it, and continued the story from there with Chicago recovering from the invasion.
 * Godzilla once did battle with S.H.I.E.L.D, the Fantastic Four, The Avengers, Iron Man, and various other Marvel Comics characters. And yes, it's EVERY bit as cool as it sounds.
 * Archie Comics Sonic the Hedgehog Super Special #17 crossed over with the entire Image Comics universe, and it's all in continuity.
 * When it was originally planned as a four-issue limited series, Transformers shared the same 616 universe as other Marvel characters. J. Jonah Jameson sent Peter Parker to Oregon to investigate the Transformers (he wanted pictures of Optimus Prime) and thus Spidey and the Autobots teamed up. When the title became a monthly ongoing series, however, the crossover was quietly forgotten. The Transformers letter column even asked fans to please ignore the earlier Marvel crossover stories.
 * A recent company crossover between IDW and Marvel did have the Avengers and Autobots team up. It's considered ambiguously canon by IDW.
 * They also, naturally, had crossovers with their G.I. Joe comic, to the point that in the "Generation 2" series, they all but merged. Years later, nearly anybody with the license will consider making their own version of the crossover, notably Image's alternate universe which is set during World War II, and Devil's Due's 4-part series, which is actually referenced in the Titanium Series toyline.
 * The original Mirage Teenage Mutant Ninja Turtles Mirage have crossed over with several other indy comic book characters, most notably Miyamoto Usagi, Cerebus, the Wild West Cowboys of Moo Mesa, and Savage Dragon.
 * Playmates' TMNT toy line even had a Miyamoto Usagi action figure at one point.
 * Not to mention Usagi's appearance in several episodes of both TMNT cartoons.
 * They also showed up in Turtles Forever, crossing over with both TMNT cartoons.
 * Heck when Archie Comics had the license, a crossover with the Riverdale gang was inevitable.
 * The Fantastic Four piloted Combattler V once.
 * To be accurate, this was when Marvel was publishing a Shogun Warriors comic book under license from Mattel, who released several Japanese Giant Robot toys under the banner. The Warriors were fully integrated members of the Marvel 616 universe.
 * Hack Slash has crossed over with several other horror series and characters, including Evil Ernie from Lady Death, Childs Play, BUMP, and Re-Animator. Tim Seeley, the writer, has said in interviews that he considers all slasher films to take place in the same universe of canon (justified by some of the entries in the cinema section of this page).
 * Star Trek / X-Men
 * The League of Extraordinary Gentlemen series by Alan Moore runs off this principle—that all of fiction exists. So you get Allan Quatermain teaming up with Captain Nemo, Mina Murray, Dr. Jekyll & Mr. Hyde, and Dr. Hawley Griffin. Then you get characters like Mycroft and Sherlock Holmes, Professor Moriarty, James Bond, Edward Prendrick of The Island of Doctor Moreau, the Martians from The War of the Worlds, Alice in Wonderland, Mary Poppins, an unnamed Dr. Fu Manchu, and just about the entire litany of late-19th century to mid-20th century fiction showing up. There are books out that detail all of the references Moore crams in there.
 * To put the above in perspective... One of the reference books for the first of the League comics has around three pages detailing one panel of all the things referenced in a museum.
 * Marvel Team-Up #74 (October 1978) teams Spider-Man with The Not Ready For Prime-Time Players to fight Silver Samurai after John Belushi accidentally gets Silver Samurai's ring.
 * In a similar vein, The Avengers once appeared on Late Night With David Letterman.
 * Image and Valiant Comics came out with Death Mate in the 90s, but the less said about that the better.
 * American Born Chinese has a subplot that crosses Journey to the West with Christianity.
 * The first Vertigo Comics Winter's Edge special has a House of Secrets framing story where Rain visits an art gallery in the house that shows her stories from The Sandman, Books of Magic, Hellblazer, The Dreaming, The Minx, Nevada and The Invisibles. While the first four are part of one continuity, the latter three are all individual continuities, as is House of Secrets itself.
 * The two House of Mystery Halloween annuals may count, because each features a story from iZombie, which hasn't been confirmed to be part of the main DC/Vertigo continuity, unlike House of Mystery and the other titles that appeared in the annuals (Madame Xanadu, Lucifer, Hellblazer and The Dreaming).
 * The ongoing (as of January 2011) JLA/The 99 miniseries.
 * Antarctic Press comics Gold Digger and Ninja High School have had frequent crossovers among their various characters.
 * Star Trek: The Original Series/Legion of Super-Heroes, from IDW Comics and DC Comics. Interestingly transplants both sets of characters to a third universe; a Legion-flavoured version of the Mirror Universe where the Imperial Planets' Starfleet wears both the Sword-and-Earth crest and the Science Police badge, and the resident Proud Scholar Race have pointy ears and arched eyebrows and green skin and blond hair. Oh, and The Emperor of The Empire? Vandal fucking Savage (Or is it an evil Flint?)!
 * Asking yourself just how he became so powerful, aren't you? Answer:.
 * And coming soon: the Enterprise NCC-1701D will be getting visited by a certain blue box. And they are going to be fighting the Borg and Cybermen. Wait, how does this series not have a page?

Fan Fic

 * Super Milestone Wars and it's sequel, Super Milestone Wars 2
 * Buffy the Vampire Slayer seems to be a bit of a crossover whore in fanfics. Crossovers include Halo, CSI, every Batman continuity, Stargate Verse, and plenty of others. A lot of the Halo crossovers are YAHFs ("Yet Another Halloween Fic", referencing the season 2 Halloween episode in which everyone got turned into their costumes) where Xander becomes John-117; these have an odd tendency to become Stargate SG-1 crossovers at the same time.
 * The online Fan Film Haloid, a truly impressive Halo/Metroid crossover. Technically, also a  crossover.
 * The ongoing crossover series by the same author called Dead Fantasy, consists primarily of stunt-laden over-the-top battles between characters from Dead or Alive, Final Fantasy, Ninja Gaiden, and Kingdom Hearts.
 * Anything by Ten-faced Paladin that isn't an Alternate Universe Fic is a crossover (sadly, a couple of them are Dead Fic).
 * There is actually a rather good one between Avatar: The Last Airbender and Harry Potter. here. He rationalizes the events as an influence for Zuko's Heel Face Turn, and Malfoy's hesitation. And it's actually partially believable as well.
 * There's a Sonic the Hedgehog/Mighty Ducks crossover (no, seriously) here which definitely qualifies, while also doubling as an Original Flavour story.
 * The Princess Bride which featured Harry as Inigo, Hermione as Buttercup, and Dumbledore as the omniscient dream narrator who broke the fourth wall repeatedly.
 * The epic Star Wars/Star Trek crossover fic Unity. For the record neither side really "wins," and for the whole second half the main character is the son of one Wars and one Trek character.
 * Trek Wars: The Furry Conflict not only has one of the worst names in fiction, but is also one of the best and longest running Star Wars vs. Star Trek crossovers out there, having been started in 1996 and continuing to this day. Several things set TFC apart from the rest are that (a) all the characters are furries with few exceptions, (b) the ST universe turned out a lot better then canon w* hile the SW universe took a decided turn for the worse, (c) the whole script is played out in a roleplay then transcribed to the net, and (d) none of the main characters from either franchise get more then a passing mention. In other words: it's a self-described Dramatic Parody that does a fairly good job of meshing the two universes, thanks to a big fat Negative Space Wedgie. It even has it's own wiki!
 * An early Star Trek fanfic (created after Season 2) crossed over with The Muppet Show with the excuse that Doctor Bob (Rowlf) was the ship's new doctor. Read the resulting hilarity here.
 * The fanfic The Chronicles of the Nine Kingdoms is essentially all the Nintendo Worlds (with a few other worlds that have appeared on Nintendo systems, like Dragon Warrior Monsters) crossed over together with the Mario world as a centerpiece for a quest against evil. The author even takes it farther by planning to cross his fanfic with a Metroid fanfiction written by his friend.
 * Fuku Fic -- Ranma ½-Sailor Moon crossovers are so common they merit their own trope.
 * Both Syllables is a fairly well-known crossover between Invader Zim, Buzz Lightyear of Star Command, and Lilo and Stitch The Series.
 * An online fanfic crossover between Star Trek: The Next Generation and Babylon 5 contained a scene where Data turned to Captain Picard and said "sir, we appear to be in a universe where we are expected to obey the laws of physics".
 * There's been a few Zero no Tsukaima stories on fanfiction.net where Louise instead summons a different familiar from another series. So far we've had Edward Elric from Fullmetal Alchemist, Chrono Harlaown from Magical Girl Lyrical Nanoha and Grimlock from Transformers Generation 1.
 * And Shirou EMIya (partial caps intentional) of Fate/stay night in The Hill of Swords.
 * Also, Jack in Zero's Shock and ALEX FREAKING MERCER in Unfamiliar.
 * There is an obscure X-Files and MST3k fanfic called The Project (link leads to a MST-ing of the fanfic). While the plot is rather generic for an X-Files fanfic, the real charm of it comes from the fact that the author seems to have seen about five minutes of MST3k, leading to Joel being emo, having people die from watching bad movies and ; it's absolutely hilarious.
 * TypeMoon vs. Godzilla. Really. It's a fanfic Novel that may be in Japanese, but the art created for it is enough to tantalize everyone.
 * The Doctor and the Enterprise, a Crossover Fanfic that predates the Internet, was eventually published with the Star Trek and Doctor Who serial numbers filed off.
 * The Sin City and Batman fanfic called A Dark Knight Over Sin City.
 * A surprising number of Harry Potter and Yu Yu Hakusho crossovers are floating around on the internet. The plot usually has the Yu Yu Hakusho cast infiltrate Hogwarts (with The Best Defense probably being the best one, and likely something of a precursor to the rest).
 * Suikoden Fantasy, a crossover of Final Fantasy VI and Suikoden III.
 * Kitsune on Campus manages to place Naruto in the Mahou Sensei Negima universe.
 * Goku/Anne Frank: Until the End of Time. If the notion of a Dragon Ball Z/Anne Frank crossover hasn't made your brain implode already, wait until you get to the part where Hitler becomes a Super Saiyan.
 * I see your Goku/Anne Frank and raise you When Wario Met Mrs. Cartman, which is, well, about Wario meeting Cartman's mom. It's also a brain-melting Mega Crossover.
 * Heero Potter, where Heero Yuy is Harry's son, Duo Maxwell is Duo Weasley, Trowa Barton is Trowa Weasley, and Quatre Raberba Winner is Quatre Malfoy. I never knew the After Colony years started sometime soon after Harry Potter allegedly ended.
 * Because of the popularity of a background character "Dr Hoof/Whooves", there are a swarm of crossovers between Doctor Who and My Little Pony Friendship Is Magic. It is even the case that many fics that aren't an actual crossover will have the Doctor as a side character and more or less explicitly make clear that yes, he is the time-travelling alien beloved by fans; he just regenerated as a pony.
 * Stars Above, a crossover between Lucky Star and Puella Magi Madoka Magica. Yes, really.
 * Battlestar Galactica Reimagined and BattleTech crossovers have a bit of popularity due to the similarity of their FTL systems.
 * "Connecting the Dots.", a intercontinuity crossover between the Justice League of America and Naruto, along with a few thousand random cultural references. Think Superman fighting Naruto.
 * Law and Bubbles, probably the only time you'll see The Powerpuff Girls meets Law and Order.
 * Real Person Fic is well-known to feature this type.
 * In The Dark has the Spice Girls and the Backstreet Boys on a joint-tour gone wrong.
 * Astral Journey: It's Complicated has a variety of singers from The Nineties as athletes, like football (soccer) in this Alternate Universe.

Film

 * Freddy vs. Jason.
 * Randolph and Mortimer Duke from Trading Places—reduced to homeless poverty by the events of that film—show up in Coming to America. They fail to notice Prince Akeem's uncanny resemblance to Billy Ray Valentine.
 * As a random gag in the third act of Waynes World, Wayne gets pulled over by a cop on a motorcycle... who turns out to be the T-1000, looking for John Connor.
 * He shows up again in Last Action Hero.
 * In Casper, the bad guys attempt to de-haunt the house using the services of Father Guido Sarducci, and Dr. Ray Stanz from Ghostbusters. "Who you gonna call? Someone else!" (According to TheOtherWiki there was also a scene filmed with Zelda Rubenstein (Tangina Barrons from |Poltergeist) but it was cut.)
 * In Little Nicky, when Nicky goes to Heaven he finds
 * Godzilla vs King Kong, in which the King of Monsters and King of Kong duke it out on Mount Fuji. It is epic.
 * George of the Jungle ends with George holding George Jr. over the very same rock where Simba was similarly held. Of course, it's all Disney. And George kicked some serious lion's ass, so it could be canon.

Literature
"(On encountering Gladiator in deep space) Kirk: Did he just punch my ship? Scotty: Aye, Captain. And he di' damage the shields a bit."
 * The officially published Star Trek: The Original Series tie-in novel Ishmael by Barbara Hambly is an extended crossover with the 1968-1970 ABC series Here Come The Brides, including several Mythology Gags spanning both series and a number of Shout Outs. Additionally, it features additional crossover elements from Have Gun — Will Travel, Doctor Who, Star Wars, Battlestar Galactica Classic, Bonanza, and Maverick, making it also a Mega Crossover.
 * A crossover introduced the crew of the USS Enterprise NCC-1701 to the X-Men:


 * And my favorite:

"Nurse Chapel: Doctor McCoy?! Beast and Bones: Yes?"


 * Ladies and gentleman, the entire comic was leading up to that. That was the whole reason for this thing.
 * There is a novel where Picard meets the X-Men (Planet X). It includes a rather awesome Hilarious in Hindsight where people point out how much Professor X looks like Captain Picard.
 * This is made even funnier by the realisation that the book was published over a year before X-Men 1 was filmed. There are apparently very few charismatic bald actors to go round...
 * Diane Duane is a Doctor Who fan, and as a result the Doctor (in different incarnations) has appeared in both her Young Wizards series and in one of her Star Trek novels.
 * Neil Gaiman won a Hugo Award for A Study in Emerald, his crossover between Sherlock Holmes and the Cthulhu Mythos. Deservedly—the story is awesome. It can be read here for free, in nifty newsprint format. Do so now.
 * Since L-Space means that libraries of sufficient size and/or complexity are interconnected, it's entirely possible for a crossover with anything. One wonders whether Havelock Vetinari and Klaus Wulfenbach would get along...
 * The basic concept of the Thursday Next books is that all novels are connected through the Bookworld, and Thursday can travel to them. For obvious reasons, the books Thursday enters are mostly public domain, but the cast of Enid Blyton's Shadow The Sheepdog, Beatrix Potter's Mrs Tiggywinkle and Kathy Reichs' Temperance Brennan have all appeared with permission. (Of the estates in the first two cases, obviously.)

Live Action TV

 * The king of the Intercontinuity Crossover is undoubtedly Detective John Munch, played by Richard Belzer, who has appeared as of Summer 2007 on ten different programs. Originally a regular on Homicide: Life on the Street, during the course of that series he appeared in three crossovers with Law and Order. When Homicide finished he went on to become a regular in Law and Order Special Victims Unit. Not only that, but he also made guest appearances in Law and Order Trial By Jury, The Beat, Paris Enquêtes Criminelles, The X-Files, Arrested Development, and even Sesame Street. And in August 2007, he filmed an episode for the fifth season of The Wire.
 * Interestingly, there's an episode of Homicide in which Munch refers to people "watching The X-Files." You could take this as a paradox, or you could figure that Munch—a hard-core conspiracy theorist—leaped at the chance to play himself in an X-Files episode. Which might explain why the interrogation room in that X-Files ep looked completely different from the interrogation room in Homicide.
 * There is uncertainty over whether Belzer appears as Munch on The Wire; the character was not named and received a generic billing. He mentioned that he used to own a bar, however, which he did in Homicide, so there's some evidence that it was intended to be Munch.
 * MTM shows did this occasionally. Carla from Cheers talked about having given birth at St. Elegius, aka St. Elsewhere, and the doctors showed up at the Cheers bar once. Sam also once directed patrons to take an ailing man to St. Elegius. Similarly, Bob from The Bob Newhart Show made reference to St. Elegius, while Bob's secretary Carol showed up on Murphy Brown as a temp for Murphy, thereby enfolding it into the autistic daydream which ended St. Elsewhere. See above for more information.
 * The TARDIS, the Living Ship from Doctor Who, has appeared in the sitcoms Red Dwarf (set in the far future) and Chelmsford 123 (set in 123 AD).
 * The Doctor and Rose showed up for a panel of Buffy Season 8, which means that aliens and demons may (if it's canon) exist in the same universe.
 * The Doctor Who/Eastenders crossover (of very dubious quality, but it was for charity) Dimensions in Time could also be mentioned, although it's actually not in either of the shows' continuities (and indeed, since then they've both featured each other as fictional programmes).
 * The Kaldor City Doctor Who Expanded Universe audio dramas written by Chris Boucher and produced by Magic Bullet are set in the titular city from the Doctor Who story The Robots of Death (also by Boucher), but feature Scott Fredericks reprising the role of Psychostrategist Carnell from the Blakes Seven story "Weapon" (also by Boucher). The A History by Lance Parkin (an un-official history of the Whoniverse) makes a valiant attempt to claim that B7 may well be set in the Whoniverse, but really it's probably best to assume he's an Alternate Universe counterpart. Or just don't worry about it. (There's also a cynical manipulator played by Paul "Avon" Darrow, who knows more about Carnell's Federation background than the Kaldorians, but let's not even get started on that...)
 * Similarly, back in the heyday of Marvel Comics' British arm, there were several crossovers between the Doctor Who Expanded Universe (or at least the DWM comics branch of it) and the Marvel Universe. The first and most notable was the Special Executive, a Time Travelling Ragtag Bunch of Misfits created by Alan Moore. Originally introduced in DWM as Ancient Gallifrey's Special Ops, they later appeared in Moore's Captain Britain, where they were mercenaries.
 * Marvel also linked Doctor Who with Transformers, by way of bounty hunter (sorry, "freelance peacekeeping agent") Death's Head. This is a borderline case, however, as the linkage only occurred when Death's Head fell through a dimensional rift from the Marvel UK Transformers universe and ran into the Doctor as a result.
 * In a more solid (possibly) example, the obscure character Octus has been stated to have a Dalek altmode. He was never seen transformed, but Word of God says that's what he turned into.
 * Gallifrey itself... or rather what was left of it... shows up in the Power Rangers Lost Galaxy episode Green Courage with Coordinates to boot, This aired in 1999, Six years before the new series, by the way.
 * It appears that the soap opera Passions has set itself up as a sequel of sorts to the classic Fantastic Comedy Bewitched. It features Juliet Mills as Tabitha Lennox, a genuine witch whose daughter is named Endora and whose parents are a mortal named Darrin and a witch named Samantha. Furthermore, Bernard Fox has made two appearances on the show as his Bewitched character, Dr. Bombay.
 * The Power Rangers in Space encountered the five Turtles from TMNT: The Next Mutation... but, uh, we don't like to talk about the latter group around here (and neither do Eastman and Laird).
 * On the other side of the Pacific, the world of the Samurai Sentai Shinkenger had an unexpected visitor in Kamen Rider Decade. The crossover is in continuity for both shows; in fact, the fact that the Shinkenger world is the only world in Decade that is the genuine, canonical universe of the show in question rather than an Alternate Universe (original worlds aside) is a plot point for the arc.
 * On that note about the Kamen Rider series, Ultraman vs. Kamen Rider is actually more of a team-up special than a fight with the protagonists being portrayed by Fake Shemps, but still awesome.
 * The upcoming special Super Hero Wars: Kamen Rider vs. Super Sentai will be bringing together 240 heroes across both franchises, and once again Decade is involved.
 * The Friends episode "The One With Two Parts" introduces Phoebe's twin sister Ursula, who was a character on Mad About You. Both had been played by Lisa Kudrow, so it was an obvious choice (to the network, at least) to make the previously unrelated characters twins. That same episode had Jamie and Fran from Mad mistake Phoebe for Ursula.
 * One episode of Mad About You itself revealed that Paul's old apartment was being loaned to Kramer from Seinfeld.
 * And later it was revealed that the series Mad About You exists in the Seinfeld universe. Maybe it's a reality show?
 * Another Mad About You episode had Carl Reiner guest star as Alan Brady, his character from The Dick Van Dyke Show. This created a bit of a paradox, because in an episode of Friends (which would seem to be in the same "universe") the characters are shown watching ''The Dick Van Dyke Show."
 * Not as much as you might think : The actual DvD show ended with Rob Petrie trying to find a buyer for his life story, and Alan Brady hinting that he might want to turn it into a show, which is so meta it hurts. So, it's possible that, in the fictional world where Rob, Laura, and Alan Brady live or lived alongside the Mad and Friends characters, Alan Brady eventually settled on Dick Van Dyke to play Rob Petrie -- brain exploding must stop...
 * Also, Chandler briefly appeared in a Caroline in The City episode, and Caroline briefly appeared in a Friends episode.
 * Ross appeared in an episode of The Single Guy, playing an old schoolmate of Jonathan's.
 * Daphne and Niles of Frasier were on an episode of Caroline In The City, too, so that links Frasier (and, by extension, Cheers) into this little sitcom world, too.
 * And since Cheers linked itself to St. Elsewhere, that drags all of 1990s Must See TV into the autistic kid's dream.
 * Martial Law crossed over onto Early Edition.
 * Early Edition also featured several regular characters from Chicago Hope in one episode.
 * Martial Law also had a crossover with Walker, Texas Ranger. The first episode takes place on Martial Law and begins with the characters standing in their station discussing a recent crime. Then a voice rings out from off-screen, saying "I hear you could use some help." And then the camera shifts to a pair of cowboy boots and slowly pans up to reveal Ranger Cordell Walker, illuminated by a bare lightbulb overhead like some pagan godling. Then he and Sammo team up to stop violent, genocidal white supremacist militia members attempting to steal chemical weapons to use to ethnically cleanse Los Angeles.
 * This is followed by an episode of Walker, Texas Ranger where Sammo visits Texas to help finish up the case. Sammo's extra-sensory and clairvoyant Chinese Daoist powers are accepted by the Rangers without a word of doubt. As Walker's long-suffering partner Trivette explains, "Around here, we call that sort of thing "Cherokee."
 * The Pretender had crossovers in paired episodes with The Profiler, and the relationship between the two characters was kept in the continuity.
 * Despite concepts seemingly custom-made for crossovers, JAG and NCIS only crossed over for one two-parter. Though NCIS didn't exist as a separate show when the crossover happened. In other words, a Poorly-Disguised Pilot.
 * JAG's Lt. Bud Roberts did appear in the first season NCIS episode Hung Out to Dry.
 * Meanwhile, Lt. Cmdr. Faith Coleman and a Navy SEAL played by Adam Baldwin appeared on both series.
 * It's not quite Archie Meets The Punisher bizarre, but the special Alice two-parter that featured a visit from Boss Hogg, Roscoe and Enos from The Dukes of Hazzard (Boss Hogg was Not-Flo Joleene's cousin) is up there.
 * ITV drama series Footballers' Wives briefly exported popular character Tanya Turner to Bad Girls (also produced by ITV) when Tanya was jailed during the plot.
 * The creators of Lost and Heroes have discussed the possibility of intertwining their shows, but are largely unable to do so because they are on different networks. However, small details have crossed over. Characters on both shows have sported identical brochures for the fictitious Gannon Car Rentals.
 * Lost has also featured a few shoutouts to Alias, though it's unlikely they exist in the same universe due to Terry O'Quinn's characters in both shows being completely unrelated.
 * Likewise, an Oceanic Airlines flight to Sydney was mentioned in passing on Alias.
 * An Oceanic Airlines ad also appeared during the pilot episode of FlashForward.
 * The disappearance of Oceanic Flight 815 was mentioned in passing on Fringe.
 * This could be because Oceanic Airlines is a generic name that a lot of shows use. The Other Wiki has details.
 * Also, the at the end of the show pretty closely resembles  Probably not intentional, but wouldn't it explain just about everything?
 * Speaking of Lost, one of Chef Robert Irvine's missions on Dinner Impossible was to cook for the cast of Lost. Though it was clear to everyone that Lost is only a television show, Nestor Carbonell did appear in character as Richard Alpert and Robert was given several Dharma food drops to work with, so it qualifies somewhat for this trope.
 * In an episode of Reba involving a court, the judge is obviously intended to be Philip Banks from The Fresh Prince of Bel Air, and is played by the same actor, despite Reba taking place in Houston and Fresh Prince taking place in California.
 * Crossing Jordan and Las Vegas crossed over so often that it probably doesn't even qualify as Intercontinuity Crossover. They certainly take place far enough apart geographically that it's plausible that they're within the same continuity. I think that the Crossing Jordan characters made one more appearance in Las Vegas after their own show had been canceled, though both shows are now finished.
 * In one LV episode, Mary made a reference to booking a conference room for Dunder-Mifflin, and Heroes and the 2008 Knight Rider series have both made references. Linderman being involved would certainly explain the Montecito's high owner turnover rate. Don't combine it with the Lost bit above, though, or your head might explode.
 * When a mentally ill patient filed a paternity suit against Dr. Kiley on Marcus Welby, M.D., the trial was resolved by Owen Marshall: Counselor at Law.
 * Picket Fences crossed over into Chicago Hope (both shows created/executive produced/written by David E. Kelly and airing on CBS), when Fyvish Finkel's character Douglas Wambaugh checked into the hospital. They also poked at ER featuring a "commercial" for the hospital that parodied the opening credits of ER and later having one character say, "We don't like to talk about that other hospital."
 * Because both series were produced by William Dozier, it was practically foreordained that The Green Hornet and Kato would make an appearance in Batman, which they did in the episodes "A Piece of the Action"/"Batman's Satisfaction", which aired on March 1 and 2, 1967, as well as an appearance in a brief "window gag" in another episode. The styles of the two shows didn't really mesh, however.
 * As for the styles of the shows not meshing, it's interesting to note that in the Hornet's "window gag" appearance (the earlier of the two appearances), Batman and Robin acknowledge his and Kato's status as heroes. In the later guest appearance, the episodes run with the standard "on police records a wanted criminal" plot device of the Green Hornet property, and entangle the Hornet/Kato and Batman/Robin in a Let's You and Him Fight standoff.
 * USA Network does this on a regular basis in their ads. If you take, say, "Monk: A Tribute" as canon, Monk, Psych, Royal Pains, Law and Order: Criminal Intent, In Plain Sight, and Burn Notice all take place in one universe. Sooner or later, all of their major series end up there.
 * Disney Channel took this to an extreme, theming two entire episodes specifically for the crossover: That's So Suite Life of Hannah Montana and Wizards of Waverly Place On Deck with Hannah Montana.
 * The MythBusters appear in an episode of the original CSI (Vegas). From the way the segment is filmed its unclear whether they are intended to be fictional lab-techs, "themselves" visiting the lab, or a figment of Nick's imagination. However they later tested the same theory on Mythbusters, using clips from the CSI episode.
 * Speaking of CSI, that show once shared a rather odd crossover with the sitcom (and fellow CBS hit series) Two and A Half Men.
 * Fringe has a particularly interesting example of this trope. The Observer, or rather, whose task it is to police inter-dimensional travel and interferences with the timeline, has managed to crossover from the Fringe continuity... into the "real world". He appeared in the audience of American Idol and has been spotted in various stadiums during Fox sports broadcasts.
 * It's unclear whether the fact that Fringe Division's job used to be called the "X-Designation" is this or a Shout-Out.
 * This one's a little older, but in an episode of The Jack Benny Program, Jack goes to court for supposedly murdering a rooster he owns. Who does he get as his lawyer? Perry Mason. Although it may not count officially, since it was All Just a Dream on Jack's part. Oh, and the true murderer?.
 * An episode of Warehouse 13 included Douglas Fargo of Eureka.
 * And an episode of Eureka included Claudia Donovan from Warehouse 13.
 * Actually, this is an example of a Shared Universe, as both shows (as well as Alphas) are part of the "Syfy Verse".
 * R. Lee Ermey turned up in Season 1 Episode 4 of Pawn Stars in full Lock n' Load gear.
 * Ally McBeal and The Practice had a two-part crossover special that started on Ally McBeal on Fox and was resolved in the next hour on The Practice on ABC, with several characters from each show appearing on the other show. Both shows were produced by David E. Kelley.
 * Susan's niece was missing when her sister Chloe overdosed, and was found by the police in New York.
 * Comic Relief frequently features entirely non-canonical Intercontinuity Crossovers. Notable ones include:
 * 1997's Prime Suspect Cracker, in which Fitz and DCI Tennyson hook up and decide that now they have a relationship they don't care who the killer is.
 * 1997 also had BallykissangelDibley, in which Geraldine desperately tried to convince a visiting Father Peter that her parishioners were normal.
 * 2009's When Janet Met Michelle, which combined all BBC Three's sitcoms about twentysomethings, as the cast of Two Pints of Lager and a Packet of Crisps invited characters from Grownups and Coming of Age to their fundraiser party. The title refers to the characters played by Sheridan Smith in Two Pints and Grownups, who did indeed meet.
 * Similarly, 2010's Children in Need telethon included Eastenders Street, in which a bunch of characters from each soap arrive at the location of the other one, including Gail Platt and Denise Johnson competing to see who has the most tragic backstory. The cliffhanger ending was that Liz Macdonald is Kat Moon's mother, in a direct parody of the revelation that Kat is Zoe's mother. See also the Doctor Who entry above for an earlier Children in Need 'Stenders crossover.

Professional Wrestling

 * New Japan Pro Wrestling has Jushin Liger, Tiger Mask, and Black Tiger, all of whom were anime characters before NJPW licensed their likenesses and created wrestlers out of them. It makes sense for Tiger Mask and Black Tiger, as the anime they originate from was about wrestling in the first place, but Jushin Liger was created for a Henshin series that takes place Twenty Minutes Into the Future about a holy warrior with the power of the liger (half-lion, half-tiger) who battles evil aliens bent on destroying the world. Makes sense.
 * When Hulk Hogan's first movie, No Holds Barred, hit theaters, the WWF thought it'd be a good idea to bring in the villain of the film, Zeus, to battle Hulk in a WWF ring. They were never really clear on whether this was supposed to be the movie character come to life, or the actor assuming the character's persona to wreak havoc (in some promos, he'd lean one way, in others, the other), but one thing was absolutely clear throughout the entire angle: it was Wrestlecrap.
 * RoboCop (yes, RoboCop!) appeared at WCW's Capital Combat 1990 to rescue Sting from a cage he was placed in by the Four Horsemen.
 * Chucky (the Creepy Doll from Childs Play) interrupted a Rick Steiner promo. Sadly, he didn't take up Rick's challenge to meet him in the ring.

Toys

 * Matt Trakker of M.A.S.K. recently re-emerged as an action figure in the G.I. Joe line, presumably because Kenner (M.A.S.K.s manufacturer) merged with Hasbro (G.I. Joes manufacturer) years ago.
 * SQUUEEEEEEEEEE-- Oh, excuse me!
 * And don't forget G.I.Joe's original Intercontinuity Crossover character, WWF wrestler Sgt. Slaughter.
 * Hasbro has also done G.I. Joe action figures for Rocky Balboa and William "The Refrigerator" Perry as team trainers.
 * And also G.I. Joe action figures for Street Fighter II. In fact, it was there when Ken got his last name, Masters, because they couldn't name an action figure "Ken" thanks to Barbie.
 * Hasbro recently came out with Transformers Crossovers, so far including Star Wars and Marvel Comics characters. They are presented as the characters driving vehicles that transform into Humongous Mecha that look like they do, ranging from a Hulkbot turning into a tank to a moon-sized Darth Vader robot transforming to the Death Star. The Star Wars ones have an advantage in that they can use existing vehicles to turn into. The Marvel ones don't have existing vehicles (for the most part... but then again, try remembering the last time you saw Spider-Man's Spider-Mobile), but this is an advantage in its own way as they can be designed almost entirely around making the robot mode look right.
 * There's also the Disney Label ones, including "Mickey Prime" and a Bumblebee Donald Duck. Doesn't seem like these Disney crossovers will be coming to the US, though.

Tabletop RPG

 * Spelljammer fan campaign: Second Unhuman War as an extension of the Blood War—that is, between Elves corrupted and manipulated by Tanar'ri and Scro corrupted (if they need more) and manipulated by Baatezu.
 * Die, Vecna, Die was an intercontinuity adventure module, intertwining events in Greyhawk, Ravenloft, and Planescape.
 * Planescape, along with Spelljammer had partly come to be to faciliate crossovers by connecting the various campaign settings—or, to put it another way, to render Die, Vecna, Die style campaign-setting crossovers into ordinary crossovers. On the other hand, Ravenloft was supposed to have a rather one-way connection to the rest of the multiverse (things could enter, but they couldn't leave), so something coming out of Ravenloft to have a crossover was rather unexpected...
 * Paranoia had one of these with Cyberpunk in 1989 with the adventure Alice Through the Mirrorshades. In true Paranoia fashion, every aspect of Cyberpunk was given a thorough mocking, from its gritty near-future setting to it's "style over substance" ethos to creator Mike Pondsmith himself.

Theme Parks

 * Several examples in the Disney Theme Parks. The biggest example is Fantasmic, where a Legion of Doom of Characters invade Mickey's mind.

Video Games

 * Garrys Mod allows you to do all kinds of crossovers with the right mods. Want John-117 to team up with Gordon Freeman and fight Saren? Have fun.
 * Mario & Sonic at the Olympic Games featured the first meeting of Mario and Sonic the Hedgehog. Since the pair were the mascots of former warring console makers Nintendo and Sega (respectively), this was something of a big deal for old-school gamers.
 * Let's not forget Super Smash Bros. which features all of the Nintendo characters meeting (though technically it's not them, they're just Gashapon figurines brought to life). From Mario, to Link, to the Pokemon. And now there's the ones not from Nintendo, such as Sonic the Hedgehog and Snake.
 * Mario Hoops 3-on-3 and Mario Sports Mix add Final Fantasy characters to the traditional cast of Mario sports games.
 * The Fortune Street series eventually added Dragon Quest characters to the game. Sony installments of the series crossed them over with Final Fantasy characters, while the Wii and DS installments mixed them up with the Super Mario Bros. cast.
 * Two games by Shounen Jump were made that put up some of their high-end mascots up against each other in the same vein as Super Smash Bros. The most popular and most varied is the Jump Super Stars game on the Nintendo DS. On the Gamecube, focusing on its most successful series, is Battle Stadium DON, with DON being an acronym for Dragon Ball, One Piece, and Naruto.
 * Wario Land Blast: Featuring Bomberman! which is pretty much what the title says. (Except in Japan, where it was GB Bomberman and had nothing to do with Wario.)
 * The ending of Bomberman for the NES had Bomberman turning into the player character from Hudson Soft's NES version of Lode Runner.
 * The Gilliam Yaeger who is the final boss of the Super Famicom game Hero Senki, and who appears in the Super Robot Wars initial continuity, and the Original Generation continuity are all the same character.
 * For that matters, the Super Robot Wars series itself is built on the idea of Mazinger Z and Shinji Ikari teaming up to beat up Char Aznable.
 * Also notable in Super Robot Wars OG Saga: Endless Frontier, Reji and Xiaomu from Namco X Capcom confirm that KOS-MOS from Xenosaga comes from their future (which was already established in the former's game).
 * Mortal Kombat vs. DC Universe.
 * And in Mortal Kombat 9 DLC, vs Freddy Krueger.
 * The Play Station 3 version of Mortal Kombat 9 had Kratos from God of War as a guest character
 * Though it's technically one series, each of the Final Fantasy games has its own world and continuity, which means Gilgamesh's appearances in various games (it's hinted he's the same character, moving through the alternate universes) count.
 * And then there's Final Fantasy Tactics, where you can send out other characters on jobs, and they may return having found the Big Bad's palace from Final Fantasy II, or the phantom train from Final Fantasy VI, or the fake Excalibur from Final Fantasy V.
 * Not to mention the entire sidequest to get Cloud in your party. The PSP port of Tactics also included Balthier from Final Fantasy XII and Luso from Final Fantasy Tactics A2. In the case of Cloud too, it's suggested that his stint in Ivalice occurs when he fell into the lifestream, hinting that it's the actual Cloud from Final Fantasy VII. Less so with Balthier and Luso, who are more or less there with no explanation. Lampshaded by Balthier himself even, who mentions that his role in the game... "feels more like a cameo role".
 * DJ Atomica, the (some would say) annoying announcer in Burnout Paradise, is the same guy who provided the commentary for SSX 3. In Paradise, he makes reference to SSX several times, suggesting it's supposed to be the same character.
 * A couple of recent Yes! Pretty Cure 5GoGo! video games have incorporated characters from the previous universes. The story implications of the iDOLM@STER knockoff are probably not significant, but the DS game has a plot suggesting they all share a universe and simply never ran into each other before (which does happen in the Pretty Cure All Stars DX movie).
 * Some Ace Combat forum goers have taken the existence of iDOLM@STER skins in DLC to suggest that those characters are animated versions of actual musical artists in Strangereal.
 * Super Puzzle Fighter II Turbo has characters from Street Fighter, Darkstalkers, and Cyberbots. There's only one character from the latter, though.
 * Capcom's "Vs. series" started life as such with X-Men vs. Street Fighter and Marvel Super Heroes vs. Street Fighter before stepping into Massive Multiplayer Crossover territory; it hasn't looked back since.
 * Capcom does a lot of crossovers. Mega Man Battle Network with Boktai. Metal Gear Solid Peace Walker with Monster Hunter. And now... Professor Layton VS Ace Attorney.
 * Shin Megami Tensei III: Nocturne features a guest appearance by Dante from Devil May Cry. In exchange for this being done, series demon artist Kazuma Kaneko designed the Devil Trigger forms of the Sons of Sparda in Devil May Cry 3.
 * One of the boons from finishing an Escape Velocity: Nova storyline is access to one of two known (very expensive) ships "found floating near a wormhole at the farthest end of known space". Scientists "theorize that these mystery ships somehow entered our galaxy from a parallel dimension." The ship is from the original Escape Velocity game, which has practically identical (albeit less developed) gameplay but a completely different universe.
 * One of the earliest intercompany video game crossovers was Battletoads & Double Dragon: The Ultimate Team, a crossover between Rare's Battletoads and Technos Japan's Double Dragon franchises (natch) released in 1993 for various platforms. This was made possible by the fact that Tradewest published both series in America and Europe, although the game itself was really more of a Battletoads game with Double Dragon characters as guest stars (some which were misnamed in the game).
 * Kingdom Hearts is a crossover between Disney and Final Fantasy.
 * My World, My Way features two characters, the mysterious dungeon diggers Owen and Kate, who were the protagonists of a previous Atlus/Global A game entitled Master of the Monster Lair. Owen even carries around the magic shovel that he had in Monster Lair, and the dungeon layouts are similar in both games.
 * So far, Link, Heihachi, Spawn, Kratos, Ezio Auditore, Darth Vader and Yoda, and Starkiller have all appeared in the Soul Series.
 * Telltale Games's latest game, Poker Night At the Inventory, features Tycho Brahe, Max, Heavy Weapons Guy, and Strong Bad, from Penny Arcade, Sam and Max Freelance Police, Team Fortress 2, and Homestar Runner, respectively, all playing poker together, plus an assortment of characters milling around the background. Plus, it could theoretically cross over with just about any property that's ever had a game made of it, period.
 * Although it's all from the same company, this schtick is a favorite of Nippon Ichi. Characters tend to show up in each other's games with reckless abandon, ranging from minor cameos to secret characters to actual plot-relevance. Laharl ends up in a bottle in Phantom Brave and chews out Revya in Soul Nomad and The World Eaters for trying to take his title. Etna is a central secondary character in Disgaea 2: Cursed Memories. Lujei (from Grim Grimoire) is responsible for Endorph (who is really ) appearing in Soul Nomad. Everyone of any importance shows up in Disgaea 3: Absence of Justice as downloadable content. And to top it off, you have Asagi, the wandering protagonist without a game of her own, trying to usurp the hero role in almost every game.
 * Interesting crossover here, in Devil May Cry. Dante receives the sword named Alastor, imbued with an electrical motif. Later on in Viewtiful Joe, a character named Alastor appears, complete with the electrical motif. The connection is later made in the PS2 port of Viewtiful Joe in which Dante guest stars and Alastor harbors some unfriendly feelings towards Dante.
 * After this, the connection is pushed forward in Tatsunoko vs. Capcom in which Morrigan's win quote vs. Viewtiful Joe mentions that she was disappointed in him after talking to Alastor. Leading to an implication that Darkstalkers, Devil May Cry, and Viewtiful Joe are all part of the same universe.
 * Free Flash game crossover: In Shift 3 you can unlock Fancy Pants Man as a Secret Character.
 * Metal Gear Solid 3: Snake Eater featured a bonus minigame where Solid Snake is assigned capture a bunch of escaped monkeys from the Ape Escape series. In turn, Ape Escape 3 featured a bonus mini-game where the player controls a monkey named Pipo Snake who must rescue Solid Snake.

Web Comic

 * The Crossoverlord. (Yes, that's the name of the comic and the no space is intentional.)
 * Now has a sequel called Crossoverkill
 * Unwinder of Unwinders Tall Comics adopted Marmaduke. Then, in the Watchmen parody comic, the big dog dies à la Jon Osterman. According to the author, "It's not Tall Comics canon, but it IS Marmaduke canon."
 * In a guest strip for Pv P, Kris Straub proposed that his comic Starslip Crisis takes place in the same universe, just thousands of years later, and his characters are descendants of those in PvP. Scott Kurtz said it's canon, though maybe he was joking.
 * Cross Time Cafe: Characters from at least five different webcomics are regular cast members - though half the cast appears more here than in their own comics (the main exceptions being Florence, Jack and Jenny.
 * Something*Positive is practically some sort of Massive Multiplayer Crossover nexus of webcomics. (See that article for more.)
 * The Cartoon Chronicles of Conroy Cat has the story take place in Cartoon Land, and has cartoon characters from different series making appearances.''
 * Powerpuff Girls Doujinshi starts with The Powerpuff Girls and Dexters Laboratory, but also brings in Justice League, Men in Black, and Megas XLR.
 * A storyline running through both Kevin and Kell and General Protection Fault had Nick and Ki literally "crossing over" to Domain.

Web Original

 * Star Wreck: In The Pirkinning, a Finnish Babylon 5/Star Trek crossover movie that took seven years to make. So good, it's apparently the most-watched Finnish film, and Universal Studios are distributing an updated version.
 * How It Should Have Ended gives their take on how the Terminator series should end. With an unexpected crossover.
 * I'm a Marvel And I'm a DC has Marvel and DC characters (and occasionally characters from other continuities, such as Hellboy) arguing with each other about whose comics and movies are best. Then there's the "After Hours" series which has actual storylines involving characters from both continuities, in the same vein as the official Marvel/DC crossovers.
 * Ghost Nappa has been in a duel, Atlantis and Lanipator and Kaiser Neko's hotel room.
 * And seeing as he had a Pokeball prior to his death, perhaps he also, at some point, visited another continuity.
 * He's also been spotted in Death City.

Western Animation
"Artie Ziff: Idiots."
 * According to the special The Grim Adventures of the Kids Next Door, The Grim Adventures of Billy and Mandy, Codename: Kids Next Door, Ed, Edd n Eddy, and The Powerpuff Girls all take place in the same universe.
 * Transformers and G.I. Joe have a long history of crossovers, which includes Inhumanoids and Jem through the shared character of sensationalist TV reporter Hector Ramirez (a Geraldo Rivera parody). The original comic books had a special crossover miniseries, and recent comics have revisited the crossover in various ways. The old eighties cartoons, meanwhile, shared a reality: in the futuristic third season of Transformers, major character Marissa Faireborn was the daughter of G.I. Joe character Flint, and one episode featured a washed-up Cobra Commander doing business under the pseudonym "Old Snake."
 * Nickelodeon's shows The Fairly OddParents and The Adventures of Jimmy Neutron Boy Genius had three cross-over Made For TV Movies: "The Jimmy Timmy Power Hour", "Jimmy-Timmy II: When Nerds Collide", and "Jimmy-Timmy III: Jerkinators".
 * Rugrats Go Wild
 * Disney took this to another extreme with Lilo and Stitch The Series, doing several specials that crossed it over with a bunch of the other shows on Disney Channel. Of course, this fits with the movie's initial marketing push, showing Stitch crossing over with various Disney movies, wrecking the place, and being told to get his own movie.
 * And, as a result, aliens are now Canon in Recess and The Proud Family... Which seems kinda odd, considering how relatively realistic their premises were.
 * Though it does somewhat fit with The Proud Family's usual weirdness. Heck, Penny's great-aunt canonically has telekinetic powers.
 * And the fact that, by extension, American Dragon: Jake Long is set in the same universe as Liloand Stitch, Kim Possible, and Recess has lots of strange implications...
 * It's very confusing with Recess, as it's made explicitly clear in the series that it takes place over the course of the Fall of 1997 until the Summer of 1998 (and then Fall 1998 for Recess: Taking the Fifth Grade), even for the episodes made in the 2000s (this is firmly established in Recess Schools Out)
 * In one episode of Duck Dodgers, Dodgers gets his dry cleaning mixed up with Hal Jordan's and ends up as the newest Green Lantern when he finds the power ring in Jordan's uniform pocket.
 * The episode name, for the curious, is The Green Loontern. And in another episode (Samurai Quack) Dodgers takes the place of Samurai Jack. Complete with Aku Achoo, voiced by Mako himself.
 * On The Simpsons, Homer's claim to having had a close encounter with an extraterrestrial is investigated by none other than Agents Scully and Mulder.
 * The Agents asked if any of the following aliens are familiar: Marvin the Martian, ALF, Chewbacca, and |Gort.
 * Writer Mike Reiss described it as the "most illegal shot" in the show's history.
 * In a move which infamously pissed off Groening, Jay Sherman from The Critic comes to help judge a film festival. An episode made after The Critic was canceled has Jay turn up again, this time confined to an insane asylum and only able to say his Catch Phrase, "It stinks!"
 * He appears again, along with other one-time characters voiced by Jon Lovitz.


 * "D-uh! You stay out of Riverdale!"
 * "We drove 2000 miles for this?"
 * "It's okay. That was Shelbyville"
 * "Just gimme another beer, you braindead hick! I'll kill you! I'll kill all of you!"
 * "Gee, Joe. You haven't been the same since your son went crazy in Vietnam"
 * Greg Weisman has written a Gargoyles/ Spectacular Spider-Man crossover radio play, although it is unofficial and non-canon.
 * There were also plans to do a crossover when it seemed there would be a series version of Atlantis the Lost Empire, called Team Atlantis. Set in the '20's, it would have involved the Atlantis cast getting involved in a battle between Demona and a former Hunter.
 * In an ad for Metalocalypse, Eddie Riggs apparently served a stint as a roadie for Dethklok.
 * Most of Hanna-Barbera's cartoons.
 * The Mask and Ace Ventura met for two episodes that could have been awesome. Both shows kept their respective styles of animation in each other's universe (Ace & Spike, for example, kept their more cartoony look in the Mask's more realistic world and vice versa).