Shared Universe

When The Verse is shaped by multiple creators, writing independently. Many different comic book titles are set in a collective continuity, making it easy to have a Crossover. In contrast, a single TV series with multiple writers is just the Verse with subcontractors. Likewise, when different continuities by the same author are tied together later by an Intercontinuity Crossover, that's Canon Welding.

The nature of the Shared Universe — multiple independent creators creating one continuity — can easily lead to a Continuity Snarl if it lasts a long time and the different creators don't take care to keep things straight. If a Shared Universe starts relying too heavily on continuity, especially if it's obscure or too reliant on each work in the Verse, a Continuity Lock Out may occur. When creators disagree on the direction the Verse should take, they may fight Armed with Canon. If some corners of the continuity are "off limits" to some characters to avoid theme-drift or plot derailing, then Superman Stays Out of Gotham.

When they go back centuries, and even further and further, long before copyrights and trademarks, the Shared Universe turns into one or more actual mythologies. Compare with The Verse, Expanded Universe and Canon. Contrast with Shout-Out.

Note: just because two or more works have had a Crossover does not mean that they share a universe.

Anime and Manga

 * A truly bizarre case is GaoGaiGar and Betterman, which take place in the same universe despite the former being a Super Robot Reconstruction and the later a Real Robot horror series.
 * Dragon Ball and Doctor Slump
 * Cromartie High School and Di Gi Charat
 * Studio Trigger has done this with their series Space Patrol Luluco, where it's established that each one of their series takes place on their own planet.

Comic Books

 * Various universes of Marvel Comics.
 * The Marvel Universe of Earth-616.
 * The Ultimate Marvel universe.
 * Similar to Marvel, The DCU is an examples of this, with multiple monthly titles who might not even have the same creative team month to month.
 * And those two are connected by the Amalgam universe, several canonical crossovers and a few characters who break the fourth wall.
 * Ninja High School and Gold Digger loosely share a universe and occasionally engage in crossovers or use each other's villains.
 * In the Savage Dragon, there is a shared universe that not only consists of the rest of the Image Universe but also creator-owned properties such as Hellboy, Madman, and Bone have made appearances. Aside from that, Erik Larsen likes to slip in characters from the Marvel Universe and DC Universe. Often, this consists of characters showing up far in the background, being mention in passing but not shown, or having a single boot or glove visible that indicates that those characters are there but enough is concealed to avoid copyright issues.
 * Virtually all of the early Image Comics titles were set in the same universe, with the stars of any given book often making guest appearances in another. However, one of the core ideas of the company was and always has been creator ownership. This caused a Continuity Snarl no less than twice; Once, when Rob Liefeld picked up his characters and left to create Awesome Comics (though he returned after Awesome folded), and again when Jim Lee took his properties, which encompassed about half a dozen titles, and made his Wildstorm Studios into a DC imprint.
 * Currently, Invincible shoulders a lot of weight when it comes to establishing a larger Image universe. Characters from Kirkman's other books popping up frequently, and big events (like the funeral of the Guardians of the Globe or the Invincible War) feature just about anyone who's anyone in the company at the time. At one point Mark was even a member of the Pact, a team consisting of him, Zephyr Noble, Fire Breather, and Shadowhawk.

Fan Fiction

 * Dangerverse fans have written numerous fics of their own set in the same universe, many of which have been integrated into the canon, as well as Alternate Universe Fic aplenty. The author has no qualms about working in ideas from her friends and fans.
 * The AU Shadowverse stories about Magical Girl Lyrical Nanoha characters Lutecia and Vivio, created by Radiant Beam, also involve many other writers who write about secondary characters in that universe. Each of the various authors tend to write around different themes (spy-thriller, emotional drama, political-thriller, etc) despite writing in the same AU.
 * More than a decade after the release of Under The Bridge, Chip 'n Dale Rescue Rangers fan fiction writers love to include enough elements from The Nowakverse into stories of their own, especially the main original characters.
 * The Secret Return of Alex Mack has spawned a shared universe called The Teraverse (also known as the "Alexverse", for the main character of the original story).

Film

 * Because Marvel Films holds the rights to all Marvel Comics characters not given to other studios (Spider-Man, X-Men, Blade, Daredevil, etc), they have managed to bridge together a single continuity family with links between the characters stories. Nick Fury makes a cameo in Iron Man referring Tony Stark to the "Avenger Initiative," Tony appears in the film of The Incredible Hulk talking to General Ross about putting a team together (also having a nod to Captain America (comics)), Iron Man II gave many hints to The Mighty Thor, and all of this will coalesce into The Avengers movie as a Crisis Crossover.

Literature

 * The Cthulhu Mythos is a famous example of this; professional fanfiction set in his world is not only published, but was also acknowledged and supported by Lovecraft before his death.
 * CS Lewis linked his world to his friend Tolkien's universe in That Hideous Strength.
 * Susannah Clarke's short story The Duke of Wellington Misplaces His Horse connects the universes of her novel Jonathan Strange & Mr Norrell to that of Neil Gaiman's novel Stardust.
 * Hang on, isn't Stardust already implied to be part of the American Gods -verse already too?
 * The Wild Cards books were designed as Shared Universe Anthologies from the ground up.
 * Border Town is a city between the "real world" and Faerie. It was originally created by Terri Windling, but Emma Bull, Will Shetterly, Charles de Lint and several other writers have written stories set there.
 * 1632 was originally to be a one-off novel, but due to favorable fan response went beyond that, later expanding into The Grantville Gazette, one of whose main goals is to give previously unknown authors a way to be published, and paid for their work. Unlike with many anthologies, the contributions from other authors affect the "main" story line works. There are very few aspects that are truly forbidden to these authors, primarily those where it would interfere with the prerogatives of Eric Flint, the series creator.
 * Thieves' World was a dark fantasy Shared Universe created by Robert Asprin in the late 1970s. It had contributors like Poul Anderson, John Brunner and Marion Zimmer Bradley and generated 12 anthlogies of short stories, seven official novels and a bunch of roleplaying adaptations before writing stopped in 1989. It preemptively dealt with Continuity Snarl with a preface framing story about an old timer talking to a new arrival in the city about how one should not believe everything in the stories one hears, as everyone spins the stories to fit their agendas, to make themselves sound more important in a good story, or less to blame in a bad one, and two people telling the same story may have wildly different variations.
 * The universe of the Bolo super-tanks has been shared by everyone from John Ringo to Mercedes Lackey.
 * The Russian Death Zone series is worked on by several known Russian sci-fi authors and is loosely based on the STALKER games. Unfortunately, this tends to create certain lapses in continuity. For example, in Andrei Livadny's novels, the Order is portrayed as a rational group that believes in the existence of an otherdimentional point known as the Node based purely on empirical evidence. In Roman Glushkov's books, they are fanatics spouting religious nonsense about the Holy Node before sacrificing themselves for the cause. It could be explained that these are different members of the Order interpreting their teachings, if they were not using the same characters.
 * The Liavek anthology series- stories by several different authors, set in and around the titular city. Apparently Liavek started out as a RPG invented by Will Shetterly for his writer's group, The Scribblies; they later fleshed out the setting and produced five volumes of short stories (and a few poems). Two of the authors- John M. Ford and Pamela Dean- later wrote more stories in the same universe.
 * The Midnight Rose collective, a group of British SF writers, published several shared-universe anthologies in the early 1990s, with settings including Temps (tongue-in-cheek superhero stories) and The Weerde (shape-shifting aliens are the source of all the world's myths and conspiracies). Contributors included Stephen Baxter, Neil Gaiman, Mary Gentle, David Langford, Kim Newman, and Charles Stross.
 * All of Simon R Green's series appear to inhabit the same universe

Live Action TV

 * Warehouse 13 and Eureka share a universe with Alphas.
 * CSI, CSI: NY, CSI:Miami, Cold Case and Without a Trace are all in the same universe.
 * After a great many hints in the various Dan Schneider shows, iCarly canonized that it exists in the same universe as Drake and Josh, Zoey 101 and Victorious.
 * The DCLAU, the Disney Channel Live Action Universe, which is comprised of Even Stevens, That's So Raven, The Suite Life of Zack and Cody/On Deck, Hannah Montana, Cory in The House, Wizards of Waverly Place, Zeke and Luther, I'm in The Band, Pair of Kings, A.N.T. Farm, and Kickin It.
 * The various seasons of Power Rangers are shown to exist in the same Universe. This was most obvious in the early years of the series, which had one, continuing storyline culminating in Power Rangers in Space. Later seasons tend to downplay this, but the various crossovers and reappearing characters establish that the universe is the same. However, Power Rangers RPM takes place in an Alternate Universe, something not made obvious until its crossover with Power Rangers Samurai.
 * JAG,NCIS, NCIS: Los Angeles and the new Hawaii Five-0 are a shared world, as evidenced by Kensi's presence in Hawaii.
 * All the Law and Order series share a world with Homicide: Life on the Street, due to John Munch.
 * The Filmways-produced Petticoat Junction and Green Acres were both set in the town of Hooterville, and characters from each gravitated to the other fairly regularly.
 * Also frequent crossovers with The Beverly Hillbillies.
 * "The Girls Of Hollywood High," the second of two Poorly Disguised Pilots (for a proposed series about Texan private detectives called Eyes Of Texas) which aired as BJ And The Bear episodes, established that this shares a universe with another Glen A. Larson series - at one point the female PIs pay a visit to the Los Angeles Coroner's Office. John S. Ragin and Robert Ito turn up as the characters they play on that particular series, but Jack Klugman, alias Quincy, is conspicuously (and given how he felt about Glen Larson understandably) absent.

Multi-Type

 * Older Than Steam: Perhaps the oldest non-mythology example is the Jianghu (literally "rivers and lakes") fantasy world in which most Chinese Wuxia books, films, TV series, etc. are set. Jianghu dates at least to the 14th-century novel Water Margin.
 * Each Dungeons & Dragons campaign setting is its own 'Verse (See the page on D&D for more information), and the associated novels have many different authors, though - like the Star Wars Expanded Universe - the writers usually have to clear their ideas through the universe's owner.
 * Dungeons and Dragons also has all of the settings linked in Planescape and Spelljammer, but those are rarely mentioned as existing except for their own continuities.
 * Given that Urban Arcana is our Earth, only with hidden fantasy elements, the Earth that Forgotten Realms canonically is connected to is probably that Earth. Planescape, at least, has a connection to Urban Arcana via a shared character that namedrops Sigil and has a way to traverse the Shadow that otherwise acts as a boundary between Urban Arcana and the rest of the multiverse.
 * The Warhammer 40,000 universe is shared by a large number of writers; the sheer scale of the setting in both space and time helps avoid continuity problems.
 * In early versions of the background, it was heavily hinted that the Warhammer Fantasy world was part of a planet cut off from the rest of the universe by warp storms, explaining the many shared elements. However, mentioning this nowadays is liable to get you bundled into a van and never seen again.
 * Word of God places Doctor Horribles Sing Along Blog, The Tick (animation) and the Venture Brothers in the same universe.
 * The Edlundverse perhaps?
 * Spells 'R' Us was started off with Bill Hart's story A Strangeness at the Frat House and then became not so much a universe but a single series of the same character in the same errant shop all ending up with customers being transformed into something.

Video Games

 * Portal shares a universe with Half-Life.
 * The Super Mario and Donkey Kong series exist in the same universe, as shown through Donkey Kong, Diddy, Dixie, and Funky appearing in Mario spin-offs, as well as Mario and Yoshi appearing in Donkey Kong Country 2. Additionally, due to first appearing in Super Mario World and Super Mario Land 2 respectively, the Yoshi, Wario Land, and Wario Ware series are also part of the expanded Super Mario Universe. The entire Shared Universe of Mario, however, is much, MUCH larger.
 * Street Fighter, Final Fight, Saturday Night Slam Masters and Captain Commando take place in the same universe. Rival Schools may also be part of it.
 * A few Final Fight characters (namely Guy, Sodom, Rolento, and Cody) have appeared as fighters in the Street Fighter Alpha series, with stages and endings featuring cameos by other characters. Andore appears in Street Fighter III under the name of "Hugo" with Poison acting as his manager. Both, Guy and Cody returned in Super Street Fighter IV. Additionally, Chun-Li makes a cameo in Stage 1 of Final Fight 2 and the portable versions of Alpha 3 features Maki as an extra character.
 * Haggar appeared in Slam Masters as a wrestler. The U.S. localization refers to him as the "former mayor of Metro City", although the original Japanese storyline actually places the games before Haggar was elected. A couple of Street Fighter characters have cameos in the Slam Masters series (such as Chun-Li, Honda, and Balrog) and the Slam Masters are referenced in Hugo's ending in 2nd Impact.
 * Captain Commando takes place in a futuristic version of Metro City. A sculpt of Mike Haggar is featured in the game as an bonus item, and Ginzu the Ninja is a future successor of Guy in the Bushin style of Ninjutsu.
 * The first Rival Schools features Sakura as a playable character (although, her blood type is different from the one given in the Alpha series). Moreover, the Nekketsu Seisyun Nikki spin-offs has Hinata studying the "Ken Masters style of Karate" and Iinchou/Chairperson learning "Saikyou-Style" through mail. On the other hand, there are a few date discrepancies according to the first game's intro and Sakura's storyline (in which she's yet to meet Ryu, placing the series pre-Street Fighter Alpha 3).
 * While not part of the main Street Fighter continuity, the Arika-developed Street Fighter EX games shares a couple of characters (Allen and Blair) with their independently developed arcade game Fighting Layer.
 * Koei's Warriors Orochi was made to confirm that it's series Dynasty Warriors and Samurai Warriors take place in the same universe but the second trailer of Warriors Orochi 3 more or less confirms Koei's other games Warriors: Legends of Troy and Blade Storm The Hundred Years War due to the presence of Achilles and Jeanne D'Arc. But it also confirms that it's business partner Tecmo series Ninja Gaiden and Dead or Alive series take place there due to the presence of Ryu Hayabusa. By extension it might also might also put it in the same universe as Halo  due to the Spartan Nicole's Presence in Dead Or Alive 4.
 * The Ultima series features references to the Wing Commander series. In Ultima I there were spaceships that in Ultima 7: The Black Gate was explained to be the spaceship of the Kilrathi. As pointed out by Spoony here.
 * Dig Dug, Baraduke (or Alien Sector if you prefer), and Mr. Driller are set in the same world, by virtue of Taizo Hori and Toby "Kissy" Masuyo being the parents of Susumu, Ataru, and Taiyo Hori (the first of the three being The Hero of the Mr. Driller series) and the events of the first Dig Dug being referenced directly in Mr. Driller (the "Dig Dug incident").
 * EVE Online and the upcoming FPS DUST 514 are part of the same universe... literally. Players will be able to accept contracts and do missions for the player-run companies of EVE Online, and even form their own corporations that EVE Online players will be able to join.
 * The Dead or Alive and Ninja Gaiden series both take place in the same universe, complete with having characters originating in one becoming plot-integral in the other. Of course, characters will change looks to match the art style of the respective games.
 * The presence of both Seath the Scaleless and Patches the Hyena seem to indicate that Dark Souls shares the same world and universe as the Kings Field series and Demon's Souls.
 * Broderbund Software tried to work the Bungeling Empire into most of its early 1980s action games. Choplifter and Lode Runner had it All There in the Manual; Raid on Bungeling Bay had it in the title but wasn't really a sequel to anything.
 * Space Harrier is set in the Fantasy Zone; several Fantasy Zone games reference it to various degrees. The culmination of this was the unreleased crossover game Space Fantasy Zone.

Webcomics

 * Webcomics tend to be chock full of Crossovers and Shout Outs, but very few non-Spin-Off comics have Shared Universes. Among those that do:
 * Something*Positive and Queen of Wands/Punch and Pie
 * More recently, Something*Positive and Girls with Slingshots
 * Also recently, Something*Positive and Penny and Aggie
 * Melonpool and the Walkyverse
 * In fact, all of the above apparently share a universe now, since characters from Something*Positive and Girls with Slingshots have recently appeared in Shortpacked!.
 * Questionable Content as well, though mainly as the odd cameo (and an implied spacewarp between the Shortpacked store and Coffee of Doom, which is actually pretty reasonable by QC standards at least.)
 * Questionable Content also shows Kim Ross, cybernetic protagonist of Dresden Codak, laughing at Pintsize's mishaps in an IRC.
 * The pre-reboot version of Zortic was also part of the Melonpool-verse. Ralph and Splink were officially half-brothers. This hasn't been mentioned since the reboot.
 * Melonpool, Ralph, Splink, and Zortic have all appeared in The Inexplicable Adventures of Bob Aparently Voluptua knows Ralph and Splink's family.
 * Neko The Kitty and Catharsis not only share a universe, they share an apartment building.
 * The Wotch and The Accidental Centaurs
 * Ryan North believes that his Dinosaur Comics and Andrew Hussie's Homestuck share a universe.
 * The [International Comic Continuity International Comic Continuity] was created specifically to make a Shared Universe.

Web Original

 * By definition, roleplaying boards such as Survival of the Fittest are Shared Universes, since each handler has his own spin on the story.
 * The Whateley Universe. Right now, there are about a dozen authors writing about twice that many main characters. And that doesn't count the Fanfic.
 * Many Internet writing circles take the form of shared universes. Some such universes have been around continuously since the mid-1980s.
 * The Randomverse contains multiple roleplays run by different people. They include The Death Series, The Insane Quest, Smile for The Camera and TV Tropes the Adventure.
 * Jason Bortz, M. S. Patterson, and Christopher Wright share a universe informally titled "The Foldspace Universe." It's the setting for Pay Me, Bug!, among others.
 * "In My World" and "Inspector Dan Rather" seem to take place in the same over-the-top-ridiculous universe, although they are told from very different points of view and rarely overlap. The only "Inspector Dan Rather" story not told from Rather's delusional point-of-view seems to confirm this.
 * Several "imprints" on the superhero fiction newsgroup rec.arts.comics.creative, notably the Legion of Net.Heroes and Academy of Superheroes.

Western Animation

 * Rugrats, The Wild Thornberries, and Rocket Power.
 * Family Guy and The Cleveland Show, along with American Dad.
 * A subtle link exists among four of Marvel and Sunbow's Merchandise-Driven 1980s cartoons — the character of Hector Ramirez, a thinly veiled Parody of Geraldo Rivera, appears in Inhumanoids, G.I. Joe, Transformers Generation 1 and Jem, though this has not been confirmed.
 * In Transformers Animated: The All Spark Almanac II, Vector Prime confirms that all these shows and C.O.P.S. take place in the Transformers generation 1 continuity.
 * Since the Looney Tunes characters are the teachers to the Tiny Toon Adventures characters, this makes an obvious example. A not-so-obvious addenum to this is that the Looney Tunes characters also made several cameos on Animaniacs and Histeria!, which therefore means they too share the same universe. Additionally, the Animaniacs characters made cameos on Freakazoid!! (and vice versa) so this also makes this part of the WB Animated Universe (as it has been nicknamed).
 * While it's pretty obvious that DuckTales and Darkwing Duck share the same universe due to sharing one of the main characters, The Legend of the Chaos God storyline in Disney Adventures also placed Tale Spin, Chip 'n Dale Rescue Rangers and Goof Troop in it as well.
 * Disney did something similar years later by placing all of its then-running shows into the same universe by giving each show a Crossover with Lilo & Stitch: The Series: Kim Possible, American Dragon: Jake Long, The Proud Family and Recess.
 * If we take Kim's inserted cameo into the "Spot the Diff" version of a Phineas and Ferb episode into account, this apparently means that Phineas and Ferb also inhabit that universe.
 * The DCAU.
 * The Cartoon Network universe was first established with "The Grim Adventures of the KND". It consists of:
 * Codename: Kids Next Door
 * The Grim Adventures of Billy and Mandy
 * Ed, Edd, and Eddy
 * The Powerpuff Girls
 * Dexter's Laboratory
 * My Gym Partner's a Monkey
 * Foster's Home for Imaginary Friends
 * Squirrel Boy
 * Arguably Camp Lazlo as well due to being part of the CartoonNetwork Invaded crossover event.
 * The CN Invaded event was one of four E En E specials that were mentioned in the E En E movie.
 * Earlier, it was established that The Powerpuff Girls and Dexters Laboratory shared a universe. While Dexter and the girls sadly never (offically) met, both parties have met The Justice Friends. Dexter himself made constant cameos in PPG to the point where it was practically a Running Gag.