When The Verse is shaped by multiple creators, writing independently, many different comic book titles are set in a collective continuity. This makes it easy to have a Cross Over. 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 Cross Over does not mean that they share a universe.
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, Firebreather, and Shadow Hawk.
Judge Dredd and its spinoffs share a universe with Strontium Dog,Rogue Trooper,Harlem Heroes and their respective spinoffs. Earlier stories also suggested that Invasion!,ABC Warriors,Ro-Busters and Flesh were part of this universe, but these have subsequently been retconned out.
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 Lunaverse is a My Little Pony: Friendship Is Magic AU where Celestia became evil instead of Luna. It started with one story by RainbowDoubleDash who promptly opened it up to anyone else who wants to write in it. Multiple writers took him up on this offer.
Lunaverse alum GrassAndClouds2 is now running a Shared Universe of his own, The Cadanceverse. It is music themed, and features Cadance as the princess (after both Celestia and Luna were corrupted), Octavia as her student, and Vinyl, Fluttershy, Lyra, and two background ponies as the other Element Bearers.
Harry Potter fanfiction often has cameos from Wizards of Waverly Place and Hellsing, due to the similar premises of the first and the second occurring in England as well. Crossovers with Percy Jackson are also not uncommon.
Naruto and Bleach X over so much that it is very common to see OC ninja share their names instinctively.
Like the Lunaverse, Life In Manehattan (aka the Manehattanverse) is an AU of My Little Pony: Friendship Is Magic, this one based on the premise that the Elements of Harmony were found some time before the pilot and taken to a museum in Manehattan; therefore, it's there that Twilight Sparkle goes for the Summer Sun Celebration, makes friends, and ends up living. The first story by the original author covers the alternate events of the pilot, after which other authors have started writing their own tales of the Manehattan 6.
After the original Pokeumans story became popular and successful enough to have its own spin-offs on the same concept, these all the share the same universe and characters from one series occasionally appear or get a notable mention in another series. Said shared universe is the world - the ''real'' world.
Many of the animated works of Pixar seem to fall into this category, mainly because of the countless references in their films and shorts that overlap with each other. The Pizza Planet Truck from Toy Story is the best example of this, which has showed up in nearly every feature film of theirs.
Note that this universe does not (yet) extend to Marvel characters such as Blade, Daredevil, Ghost Rider and The Punisher who were under license to other studios but whose film rights have now returned to Marvel.
Other film companies are following Marvel Studios' lead, creating their own cinematic universes for comic-book properties they own the film rights to. Warner Bros is creating a cinematic universe for all their DC Comics properties, beginning with the film Man of Steel. Fox is setting their new Fantastic Four films in their established X-Men film continuity, effectively making a rival cinematic universe of Marvel properties.
Supergirl takes place in the same continuity as the Christoper Reeve films. (Jimmy Olsen appears in the film for example.)
Pulp Fiction and Reservoir Dogs, both Tarantino films, share a link in that Vincent and Mr. Blonde are apparently brothers, although it's not immediately obvious from watching the films.
Originally the connection was intended to be much stronger. In early drafts of the script, the briefcase Vincent and Jules retrieve contain the diamonds stolen in Reservoir Dogs. Additionally, a prequel to both movies, set before Vic is sent to prison in which he visits Vince in Amsterdam, was in Development Hell for years before Tarantino finally gave up because John Travolta and Michael Madsen had aged too much to convincingly play younger versions of themselves.
Additionally, Lee Donowitz from True Romance is the son of Sgt. Donny Donowitz from Inglorious Basterds. Furthermore, Alabama from True Romance is the same Alabama that Mr. White mentions in Reservoir Dogs. Meaning all four movies are in the same universe.
Also, Kill Bill is a Show Within a Show. It's a reworking of the failed TV pilot Fox Force 5 mentioned in Pulp Fiction. The actress playing The Bride is not just Uma Thurman, it's Uma Thurman as Mia Wallace.
Machete and Spy Kids share the same universe as confirmed by Word of God, one must wonder which came first in terms of timeline, the misadventures of Uncle Machete or the Spy Kids?
Bordertown 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 at professional rates instead of less generous fanzine ones. 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, with a short revival in the early 2000's. 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, originally by Keith Laumer, 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 S.T.A.L.K.E.R. 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.
Other major differences involve the very nature of the Zones. For example, in Livadny's novels, there is no plant or animal life in the Zones, as anything exposed to the scorgs gets "upgraded" (i.e. it becomes a weird mix of flesh and metal bearing little resemblance to the original). Rust by Aleksey Kalugin shows the Moscow Zone full of plant and, occasionally, animal life with only a few examples of bio-tech mixes. Kalugin's nanobots (the word "scorgs" is never mentioned) only care about consuming metal and reanimating machines.
The Liavek anthology series- stories by several different authors, set in and around the city of Liavek. Apparently the setting 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.
Merovingen Nights was an anthology series set on the planet Merovingen, in an islolated corner of C. J. Cherryh's Alliance/Union universe.
Heroes in Hell was an anthology series with a concept similar to Riverworld: all the dead wind up together in Hell, where they pick up where they left off when still alive.
The universe of the Malazan Book of the Fallen was jointly created by Steven Erikson and Ian Cameron Esselmont, and both authors have written their own novels for the setting.
The '80s Magic in Ithkar anthologies were more of a shared setting; all the stories started with the setting of Ithkar Fair, detailed in the anthology prologues, but other than that each author's stories were free-standing, sharing no characters, events, or settings beyond those established in the prologues. Most notably, one story ended with the Fair being shut down due to plague; this was not reflected in any of the others.
The Spore Wiki Fiction Universe began life as a Spore fan fiction continuity but eventually separated itself. It's shared between multiple writers and is open to anyone willing to write for it.
The Kane Chronicles contains several hints that it takes place in the same world as Rick Riordan's other series, Percy Jackson and the Olympians. This doesn't effect either series much, mainly because each is subject to the other's Masquerade, but the crossover story The Son of Sobek implies that this is changing.
Friends shares a universe with Mad About You due to Phoebe and Ursula being twins. Additionally Friends also had a spin-off called Joey, which is now part of that universe as well.
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.
"The Girls Of Hollywood High," the second of twoPoorly 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.
Most of the series produced by Jack Webb take place in the same universe, including Dragnet, Adam-12, Emergency! and the short-lived Robert Conrad vehicle, The D.A.
Arguably Andy Barker PI as well because Guy Halvorson was the owner of Doublemeat Enterprises, which Buffy previously worked for.
Once Upon a Time got a Spin-Off in Once Upon a Time in Wonderland. Given that Wonderland starred a protagonist from Victorian England (Alice), whereas Storybrooke had a protagonist from modern-day NewYork, it was up in the air how connected the two series would be. However, the writers decided to pull a big ol' Timey-Wimey Ball and have major characters interacting with one another across the two series and times. The Knave of Hearts from Wonderland is first seen breaking into Granny's diner in Storybrooke before somehow going to Victorian England to convince Alice to break out of the insane asylum she's being kept in. Then in the third episode, he's shown as one of Robin Hood's (a significant character in the third season of OUAT) Merry Men.
Cheers had one direct and well known spin-off in the form of Frasier, but both shows also shared the same universe with Wings, which was created by former Cheers showrunners David Angell, Peter Casey and David Lee, who would subsequently go on to create Frasier. While several Cheers characters appeared in both Wings and Frasier, there were never any cross-overs between the latter two shows.
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 WarsExpanded 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 tonote And hilariously so: among other things, the first edition of the Forgotten Realms Campaign Setting was canonically written with the help of Elminster 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.
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.
Safe Havens and On The Fastrack, both comics done by Bill Holbrook, take place in the same universe and have on occasion crossed over with each other (the major point being Fastrack funding the mission to Mars Samantha of Safe Havens is planning). It's implied that Kevin & Kell is a parallel universe connected to their universe, but it's unlikely to be confirmed since K&K is under a different syndication, therefore little chance of a Cross Over.
Grand Theft Auto and Manhunt (both developed by Rockstar Games) take place within the same universe, as confirmed by overlapping references like place names, vehicles and fictional brands. Rockstar's Bully is also a possibility though there are not enough references that verify this. Although the HD series exists in a different universe then the Trilogy, both Carcer City and the Bullworth Academy were referenced in GTAV.
The Super Mario Bros. 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 WarioWare series are also part of the expanded Super Mario universe. The entire Shared Universe of Mario, however, is much, MUCH larger.
In addition to this, Rare's games share a universe with Donkey Kong (as caused by Diddy Kong Racing having Banjo and Conker in it and references to Donkey Kong and Mario in their other games in the Nintendo 64 era). This not only adds Banjo-Kazooie and Conkers Bad Fur Day into the Mario universe, it leads off into what DK Vine call the DKU, where the presence of Roystein the Goldfish in yet more games integrates in Viva Pinata and Grabbed by the Ghoulies, where Donkey Kong in Punch-Out!! integrates in Punch Out and where Mr Pants can be linked to Mario by way of proxy via two franchises. Or where Star Fox apparently exists alongside Donkey Kong thanks to Tricky the Triceratops... It's a really, really confusing situation, made more complex by Rare getting bought out by Microsoft.
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. Hugo and Rolento returned in Ultra Street Fighter IV with newcomer Poison.
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).
In Night Warriors: Darkstalkers Revenge, Felicia claims she wants Blanka as the leading man in her new movie. Among various Capcom characters, Morrigan, Felicia, Hsien-Ko, Mei-Ling, and Lord Raptor can be seen on Ken's stage◊ in Street Fighter Alpha 2, although there's a distinct possibility these are very convincing cosplayers.
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.
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 King's Field series and Demons Souls.
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.
Marathon takes place in the same universe as Pathways Into Darkness, and possibly Halo. There is even a terminal in the first Marathon game (which is a historical record) that talks about the events of Pathways.
Oliver Garneau from Black Flag'sFrame Narrative leaves for Chicago (Where Watch_Dogs is set) during the game. In Watch_Dogs, Aiden kills him as part of an optional side mission under mysterious orders from"The Brotherhood". There are also several references to Abstergo Industries, and, at one point, a child can be found playing Assassin's Creed III: Liberation, which is a video game produced by Abstergo Entertainment in-universe.
Far Cry is in on the action, too. One mission in Far Cry 3, which discusses "strange scientists" and "genetic memories", takes you to an abandoned Abstergo laboratory. Far Cry 2 is definitely set in the same universe as it's sequel, but whether the original or the upcoming fourth entry are still in the universe have yet to be seen.
Touhou, Seihou, and Uwabami Breakers take place in the same universe, with the first being the only place that retains magic in the 'verse. If a certain ending is any indication, it also shares universe with To Heart.
Mega Man (Classic), X, Zero, ZX, and Legends, all on the same timeline despite the tone reaching further and further away from the original series with each new series.
Project X Zone is a Massive Multiplayer Crossover so it deals with characters from multiple different universes together, seemingly averting this trope. But on the other hand, some characters from separate series entirely are aware of each other, suggesting that games from different companies entirely might share the same universe.
Street Fighter appears to be in the same universe as Rival Schools, Tekken, Fighting Vipers and Virtua Fighter by virtue of characters apparently not only hearing of each other, but actually knowing each other and being aware of other fighting game tournaments. They're also aware of other characters not from the same genre by virtue of being aware of current events, especially Chun-Li, so they're also in the same universe as Resident Evil, Dead Rising, Zombie Revenge (and House of the Dead by extension) and Die Hard Arcade. Then as it turns out, the Makai exists in this same Earth, meaning that they all take place in the same universe as Darkstalkers. Who are aware of the exploits of Sparda, extending the universe to Devil May Cry. Dante idolizes Arthur as a legendary demon hunter, making Ghosts N Goblinsalso part of that very same universe. The demons also seem to be aware of Valkyrie from Legend of Valkyrie. At this point, why have a plot about different universes coming together?
That's not the only shared universe, though. Games from alternate histories and alternate futures are part of separate timelines, for obvious reasons. Tron is aware of the existence of X and Zero, confirming that Mega Man Legends is in the same universe as Mega Man X just like they do in the original canon. But Devilotte of Cyberbots appears to know of Tron, putting that game in the same universe, and then KOS-MOS also appears to know of X and Zero, possibly putting even Xenosaga in the same universe as the Mega Man games.
Even further, the two shared universes may actually be one due to the fact that Kite and Blackrose of .hack are surprised that they're walking around the real world using their MMO characters. Said real world being the one that all the Fighting Game characters are a part of. Then when they end up back in the MMO along with everyone else, they try exiting and end up in Cyber Peacock's stage which was apparently where some of the MMO's servers are located. And on top of that, Neneko of Yumeria recognizes Kite and Blackrose on sight, adding one more game series to the Shared Universe. The number of games that aren't part of it at this point are few.
Project X Zone is a continuation of Namco × Capcom (and Endless Frontier by proxy), which already established that, if only for the sake of that crossover's continuity, all involved series exist in the same universe, only separated by varying layers of time and space. As a large part of the NxC cast returned for Project X Zone, a few of these instances can be chalked up to "X and Y character already met." There's also no word on if other previous crossovers have any bearing on the game (the Marvel vs. Capcom series and Tatsunoko vs. Capcom in particular, where many of the characters here already crossed paths or were at least aware of worlds other than their ownnote one such example being Morrigan knowing of Viewtiful Joe due to a past correspondence with Alastor, who is confirmed to be the spirit of the Devil Arm of the same name from the original Devil May Cry in the PS2 port of Viewtiful Joe), although there may be support for the idea in-game: Bahn recognizes Akira and Pai due to Sega's own non-canon crossover, Fighters Megamix.
In fact, all of the above apparently share a universe now, since characters from Something Positive and Girls with Slingshots have 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 - it's since been established that the coffeshop near Shortpacked which has the same layout, staff, clientele and espresso dinosaur as Coffee of Doom is called Coffeeright Theft).
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.
Eerie Cuties/Magick Chicks and Vampire Cheerleaders/Paranormal Mystery Squad take place in the same universe, as evidenced by Steph and Layla's bonded hairpins and Word of God. However, the Delacroixes and Lori's coven are different breeds of vampire following slightly different rules, and Charlotte is a very different kind of witch than Mel's coven, who for as-yet-unexplained reasons are considered cryptids while Charlotte (like the Artemis Academy's magical cadets and espers) is human.
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 Fan Fic.
Many Internet writing circles take the form of shared universes. Some such universes have been around continuously since the mid-1980s.
It goes deeper in that in some cases. 3rd-season Autobot ally Marissa Fairborn has been explicitly stated to be the daughter of Joes Flint and Lady Jaye, and an episode of the same season featured a villain called "Old Snake" who was clearly Cobra Commander.
In Transformers Animated: The AllSpark Almanac II, Vector Prime confirms that all these shows and C.O.P.S. (In the toyline, the hero Checkpoint is a descendant of Beach Head) 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.
And since Pinky and the Brain also happen to share the same universe as the above shows, they're counted as part of it as well.
Word of God confirms that Transformers Prime and Transformers Rescue Bots take place in the same universe. This was finally confirmed in-universe by the Rescue Bots episode "Bumblebee To The Rescue". It's also the reason Rescue Bots is set on an island and Prime is set in the American southwest. This allows the two shows to reference each other and interact while still being seperate.
All three current Marvel Animation shows, Ultimate Spider-Man, Avengers Assemble, and Hulk and the Agents of S.M.A.S.H., seem to take place in the same universe. Broad Strokes aside, they seem to take place at different times given how Hulk speaks differently in each of them (from Hulk Speak in Ultimate Spider-Man to speaking in complete sentences in Agents of SMASH, with a transitional speech pattern for Avengers). At the very least, they have similar head writers (Paul Dini, Man of Action) and share the same voice actors for their vast amounts of characters across the shows (ie, Drake Bell as Spider-Man, Adrian Pasdar as Iron Man, Fred Tatsciore as Hulk, and Travis Willingham as Thor).