Friendly Fandoms

The opposite of Fandom Rivalry. Two (or more) fandoms have a lot of overlap because of shared talent (sometimes the result of a Production Posse), because of similar subject matter, or for no real reason at all — they just like each other! Often people get into one fandom and are encouraged by this relationship to get into the other.

Inter-Media/General

 * In spite of the oft-hyped tensions between fans of Star Wars and those of Star Trek, truth is, outside of exceptions, the two sagas are so different, they don't really step on each other toes. And in the modern times, true pop-culture geeks understand the economics of how TV and film projects get the go-ahead or not, therefore, fans of both works tend to look at the others' success as being a boost to their own favorite.
 * Due South and Hard Core Logo. Because Canadians have to stick together, eh?
 * There's a great deal of overlap, probably unsurprisingly, between fans of Horatio Hornblower, Aubrey-Maturin, and Pirates of the Caribbean. (And possibly also Temeraire and Jane Austen).
 * Doctor Who and My Little Pony Friendship Is Magic have developed some harmony between each other, mostly due to a fan-favorite minor character in the latter show reminding everybody of the Tenth Doctor.
 * The Bronies have developed friendships with a lot of fandoms in general, but it seems that the biggest one is the friendship between the Valve fandom. Mostly because the Valve games helped to spread the word on it's early days, specially Team Fortress 2. Many Team Fortress 2 crossovers have been made, to the point of having designed characters from the show to represent the characters from the game in the crossovers, and many references to the game in the pony-related videos in YouTube.
 * Helped by the steadily growing number pony models for Garry's Mod, which is used to create most TF 2 fanvids.
 * Mystery Science Theater 3000 fandoms. This gives the Transformers fandom a healthy dose of self-awareness about itself.
 * You can expect many Touhou fans to be Puella Magi Madoka Magica fans and vice versa. Especially if they cosplay.
 * Rival fandoms who begin to see each other as worthy opponents can develop into this over time. The modern-day rivalry between Mario and Sonic the Hedgehog seems to be developing into this, for example.
 * Many Alternate History Dot Com users are fans of Code Geass, due to the series being set in an alternate history. A good deal of timelines on the site are also Code Geass fanfics.
 * With the advent of the new Doctor Who, there seems to be a deliberate effort underway to unite Who fans and Star Trek fans. Besides little things like Patrick Stewart being a big Who fan and Freema Agyeman and John Barrowman being Niners, there's also Stewart and David Tennant appearing in an Old Vic production of Hamlet. And now, there's an official crossover comic from IDW publishing pitting the Enterprise-D and the 11th Doctor against a Super Villain Alliance between the Borg and Cybermen.
 * Sherlock and Cabin Pressure, due to the shared presence of Benedict Cumberbatch.
 * Western Animation fans and Anime fans are beginning to get along pretty well. Several Western Animation characters are even drawn in Anime style on Deviant ART, most notably the South Park characters.
 * There are several Homestar Runner/Penny Arcade crossover artworks and fanfictions on Deviant ART, especially thanks to Poker Night At the Inventory.

Anime & Manga

 * Basically any series by CLAMP has a significant fan overlap with any other series done by the same group. The fact that CLAMP wrote a Massive Multiplayer Crossover, Tsubasa Reservoir Chronicle, that included characters from almost every series they've ever done essentially created a pan-series fandom.

Literature

 * Terry Pratchett fans and Neil Gaiman fans, because of their collaboration on Good Omens.
 * And Douglas Adams fans overlap with both!
 * And Douglas Adams fans lap over with Doctor Who fans, since Adams was a writer and producer for the show, and one of his books, Dirk Gently's Holistic Detective Agency, was basically a rewrite of an unused DW script. Then Neil Gaiman wrote a Doctor Who episode, showing his love for the series, cementing these relationships.
 * Back in perhaps 2005 or 6, there was a major crash on a William Gibson message board, so the members of that board were essentially invited to hang out on the official Gaiman forum until the site issues were resolved. They did, and the two fandoms got along so well that some members wound up participating on both boards for years.
 * The Chronicles of Narnia and The Lord of the Rings, due to both being fantasy written by authors who were close personal friends and both had an intensely Christian worldview (which influenced their works to varying degrees).
 * Back when both series were originally coming out, the Sweep and Circle of Three fandoms fed into each other, since they were both YA series about teenage witches coming out around the same time. There were many GeoCities sites dedicated to both.

Live Action TV

 * Jeeves and Wooster and A Bit of Fry and Laurie, due to the shared double act of Stephen Fry and Hugh Laurie.
 * Joss Whedon's shows sometimes seem like they just have one big fandom.
 * J.J. Abrams' shows have this, too.
 * Veronica Mars fans overlaps with Joss Whedon fans as well, likely because Joss himself was a fan of Veronica Mars (even making a caemo once) and because it features a snarky petite blonde title character who's a strong woman.
 * Tin Man and Alice - mostly because of their similar production staff and tone.
 * Likewise with their literary ancestors. Baum was a fan of Alice in Wonderland.
 * Old-school Doctor Who and Blakes Seven were made by the same people and shared several actors and sets and locations, so B7 pretty much became the original adult sister show to Doctor Who. Most B7 fans are also avid Doctor Who fans.
 * Sherlock and Doctor Who have a bit of overlap and crossovering going on (Sherlock overlapping more with Doctor Who than the other way round, due to a smaller fanbase) thanks to The Grand Moff being executive producer at both.
 * Ever since Neil Gaiman wrote the very popular Doctor Who episode "The Doctor's Wife" for series 6, one which confirmed old fan theories, and contained a general display of awesome- the two fandoms have definitely gained a friendship.
 * Let's just say the running gag Inspector Spacetime was very well received by both Community fans and Doctor Who fans.
 * Smallville and Supernatural fandoms have a very, very large amount of overlap, to the point where it could quite reasonably argued that they share almost the same core base of fans. The fact that during the second half of the 2000s they almost always aired one after the other on the same network certainly helped this.....as did the fact that they were the two powerhouse Sci-Fi series that were basically keeping the ungrateful CW Network afloat. The then-current network president's perceived treatment of the two shows as NetworkRedHeadedStepChildren (in favor of promoting shows like Gossip Girl or the 90210 remake as being the network's flagship programs) led to further unity between the two fandoms, which rightly or wrongly perceived themselves as being united against a common foe (namely, the perceived indifference on the part of the execs).
 * The massive prevalence of fanfic crossovers between the two shows is also a testament to the unity between the Smallville and Supernatural fanbases.
 * Power Rangers and Super Sentai are two fandoms you'd expect to get into some variation on a Subbing Versus Dubbing brawl. And yet, that does not happen. Both the fans and production crews get along extremely well (to the point that they share sets and props, and for a couple of Super Sentai opening and ending sequences in the past few years, they filmed in New Zealand using the crew for Power Rangers), and forums for one will usually allow discussion of the other...
 * ...but on the other (sad) hand, YouTube monkeys play Fandom Rivalry in an utterly straight manner.
 * This joint fandom can be expanded to include other Toku to various degrees; including Kamen Rider, Metal Heroes, and their various American adaptations (except maybe one we'd like to forget). The Japanese production house, Toei, has been encouraging this in recent years by including crossovers as part of their Milestone Celebrations.
 * Out of the prominence of both fandoms on Tumblr, SuperWho was born.
 * Stephen Colbert and Jon Stewart seem to both share the same fandom. Makes perfect sense, since Stephen used to work on Jon's show, and they interact frequently. It's almost like The Daily Show and The Colbert Report are one show with two different hosts.
 * The Band of Brothers, The Pacific, and Generation Kill fandoms, whose fans are collectively known as the "HBO war fandom".
 * The King of Queens and Everybody Loves Raymond take place in similar settings and had frequent crossovers, and thus share largely the same fanbase.
 * Interestingly, Food Network shows generally have a lot of overlap in their fanbases, but not completely. While they are largely friendly towards one another, one celebrity chef's fans will often complain that several others on the same network are "too grating" or annoying.
 * There is generally a lot of overlap between fans of TNT's original shows.
 * Also thanks to Tumblr, we get SuperWholock
 * Star Trek fans are known for their running feuds with fans of other Sci-fi series/franchises; notable rivals being Star Wars, Babylon 5, and Doctor Who. But it seems there's a lot of warm regard, if not necessarily love, for fans of the televised Stargate shows especially Stargate SG-1. As different as the shows are, their similar takes on military science fiction/SpaceOpera and comparable places on the Sliding Scale of Idealism vs. Cynicism naturally means they'd appeal to a similar group. Also, it's clear the producers of Gate pay homage, if ribbingly, to Trek. To top it off, throw in the number of Trek actors who've would up on one or the other Gate series and it's almost impossible to see this fandoms as rivals.
 * For a long time on Sci Fi Channel, The Next Generation and SG 1 were both staple shows, sometimes airing within hours of each other.
 * Star Trek and 24 fandoms got pretty cozy what with all the former (and sometimes current) Trek actors appearing in the show in big and large parts. The connection was especially strong with Star Trek Enterprise which saw Gregorty Itzin and Peter Weller doing both shows at the same time. (Itzin as President Logan on 24 while playing an alternate universe Starfleet Admiral on Enterprise. Peter Weller portrayed a Well-Intentioned Extremist on both shows!) The biggest connection was when Show Runner Manny Coto left Enterprise after it was cancelled and became a producer on 24.

Music

 * Most jazz and classical music fans and players will like each other and go to each other's concerts.
 * Bandom and Popslash, for somewhat obvious reasons.
 * Yuki Kajiura and Yoko Kanno have both this and Fandom Rivalry, depending upon the specific persons talking.
 * Fans of the Foo Fighters and Queens of the Stone Age seem to get along well, since their front-men are involved in several projects together (Dave Grohl has even guested on a few QotSA songs) along with being poster-boys for modern alternative rockers' habit of having too many damn side-projects.
 * Nightwish and Sonata Arctica share a lot of fans, probably because the bands have toured together and the members are friends.
 * Mr. B The Gentleman Rhymer and Professor Elemental have both this and Fandom Rivalry, depending upon the persons talking.
 * The fandoms of Tori Amos, Bjork, and PJ Harvey have a lot of people in common. It's probably because they all started their solo careers during the early '90s and were in an interview together.
 * Otep, Amanda Palmer and Emilie Autumn. They're apparently all friends IRL. Also, to some extent, Courtney Love and Emilie Autumn(due to the fact that Emilie was in Courtney's backing band), but that sometimes turns into Fandom Rivalry instead.
 * Tupac Shakur fans and The Notorious B.I.G. fans used to not get along. Then, 1997 came around with both their deaths, and they are now far more amicable on account of Dead Artists Are Better taking effect.

Professional Sports

 * New York metro area baseball and football fans generally divide in the following way: fans of the New York Yankees baseball team generally are also fans of the New York Giants football team, and vice versa; meanwhile, the Mets baseball fans are generally also fans of the New York Jets football team. The intra-city fandom rivalry between the Yankees and Mets and the intra-stadium rivalry between the Giants and Jets helps add to this, as does the fact that before Giants Stadium was built, the Yankees gave the Giants a home at Yankee Stadium, while the Mets played host to the Jets.
 * It should also be noted that the Yankees/Giants fans and the Mets/Jets fans in New York City are generally united in their mutual love for the New York Knicks basketball team. However, the Yankees/Giants and Mets/Jets fans in NEW JERSEY often root for the New Jersey Nets basketball team. Mets/Jets fans in Jersey who support the Nets, often proudly proclaim their love for "Mets, Jets, and Nets!"
 * Also, Yankees/Giants fans tend to root for the Rangers hockey team, and Mets/Jets fans support the Islanders.

Theater

 * In the Wicked section it's almost heresy not to have seen Rent or at least know the basic plot. It's probably something to do with the large number of shared actors especially Idina Menzel but there have been Wicked actors playing Rent, Rent actors playing Wicked, crazy crossovers, songfics with songs from the other musical and just odd references in fics.

Video Games

 * BioWare fandom doesn't seem to care which game is better or not. If you find someone writing fanfic for Shepard, chances are they'll also have a Revan, and maybe a Bhaalspawn or Spirit Monk.
 * As long as nobody criticizes Baldur's Gate...
 * Ditto with the few and proud fans of Sierra. King's Quest, Space Quest, Leisure Suit Larry, Quest for Glory... It's "Yes, please" not "which is your favorite?"
 * In the Console Wars, Fans of First and Second Party franchises of the console they are representing tend to have a good relationship with each other and are even fans of each other’s games. Such examples include Halo and Gears of War for the Xbox front, Super Mario Bros and The Legend of Zelda in the Nintendo front, and Killzone and Uncharted in the Playstation front.
 * Know someone who plays Tetris the Grand Master? Chances are they also play Bemani games and/or shmups. Conversely, Bemani fans and shmup fans view TGM as a new challenge that's worth giving a shot, instead of just crazy insane invisible antics.
 * DJMAX (or at least DJMAX Technika) and Bemani. Many Technika players have a history of playing Bemani games.
 * Strangely, the Fighting Game fandom, in some regions, has started to blend in with, of all things, the Catherine fandom. At many fighting game events, you can expect to see the host running a tournament for Catherine's versus mode. Who knew that block climbing would become as popular as 20-hit combos?
 * Alot of fans of Namco's franchises (Pac-Man, Tekken, Mr. Driller, Klonoa, etc.) are also fans of Capcom's franchises (Mega Man, Street Fighter, Ghosts N Goblins, etc.) Especially since there's Namco X Capcom, Street Fighter X Tekken, and Tekken X Street Fighter.

Webcomics

 * Dinosaur Comics and Xkcd. Even their creators are friends!
 * But Ryan North is a computational linguist!
 * You can also count Andrew Hussie and David Malki! as creator friends with friendly fanbases.
 * Speaking of Hussie, the Dinosaur Comics and MS Paint Adventures fandoms get along rather well, considering that the official forums of the former are a section of the fora of the latter. This is probably because, again, the creators are very good friends, and because of a very similar style of humour.
 * 910Comics (The Wotch, The Wotch: Cheer, Misfile, etc.) readers used to have insane amounts of overlap with El Goonish Shive Bunnies. Until El Goonish Shive joined the 910Comics community, and now they're the same fandom. Same with Zebra Girl, and probably others as well.

Web Original

 * The two superhero parody shared universes of the early days of the Internet, Superguy and Legion of Net.Heroes. At least back when Superguy was active. They shared a couple of important writers in common, such as Mike Escutia and Dave Van Domelen.

Western Animation

 * Many of the prominent Fanfic writers for Adventures of the Galaxy Rangers also wrote fanfic for The Real Adventures of Jonny Quest back in the 1990's. These days, newcomers to the fandom tend to be fans of X-Men: Evolution who discovered it via Red Witch and other fanfic writers.
 * G.I. Joe and Transformers fans usually get on well.
 * My Little Pony fans also get along with with Transfans and Joefans. Although there is some friction caused by the more outspoken Bronies.
 * Likewise, many fans of Dan Vs. also get along well with the MLP fanbase. Both shows are on The Hub as it is.