A crossover that involves characters from more than two works of ficton. More often than not, this is a mash up of series which do not have a strict sense of continuity or a clear Universe Bible. To lessen canon-faulting, especially with series that do have strict continuity, a new 'neutral' setting is made that offers equal footing for all the characters. It also becomes more viable the farther you get from canon, such as one-time TV specials and especially video games. As Story Arcs have become more prevalent, this practice has somewhat lessened, with shifts to strict Verse building and explicit references.
This rarely occurs in live action shows, unless a production company can be formed that holds copyrights to everything. Thus, this is much more common in animated works — although you can generally expect The BBC to pull one out of somewhere when Children in Need or Comic Relief rolls around.
This trope has become increasingly common in video games, especially those involving both licensed and original properties. These games, depending on how far or how deep they mine, can have interesting effects on the fiction chosen. Many long-gone and/or forgotten Humongous Mecha shows, for example, often get a new lease on life, or even a brand-new sequel or remake, after making an appearance or two in a Super Robot Wars game. Similarly, the Fire Emblem series was finally brought over to the US to great success after two of its characters made an appearance as unlockable fighters in Super Smash Bros. Melee.
This trope is Older Than Feudalism. The Argonautica by Apollonius Rhodius (3rd century BCE) features nearly every ancient Greek mythical hero all going on a quest to find the Golden Fleece.
The Giant Robo OVA series featured characters taken from several other series Misuteru Yokoyama — the original creator of Giant Robo/Johnny Sokko and His Flying Robot — had written. This included adaptations Romance of the Three Kingdoms and The Water Margins, which led to many main and secondary characters in ancient Chinese clothing coexisting with people in three-piece suits Twenty Minutes into the Future. It also included the very first Magical Girl, Mahotsukai Sally (Sally, the Witch), under her original name "Sunny", as Shockwave Alberto's daughter.
Their first crossover happened in Clamp Campus Detectives since the three main character were in series/stories on their own before CCD: Nokoru Imonoyama in Duklyon, Akira Ijuin in 20 Mask ni Onegai (Man of Twenty Masks) and supposedly Suou Takamura showed up in an old oneshot. Then there was X/1999 where characters from past CLAMP series started appearing, including Subaru, the main character of Tokyo Babylon, as a major character, and during his series set in the early 90s it was said that he would have a role in The End of the World as We Know It that is X, making for a bit of foreshadowing.
Before Dengeki Gakuen RPG: Cross of Venus (mentioned below), there was this animated short produced for Dengeki Bunko's 2007 Movie Festival, featuring chibi versions of characters from Kino's Journey, Inukami! and Shakugan no Shana (Note that the chibi Shana here is notShana-tan; for one, she is the stalker rather than said omake series' Kazumi.).
Crisis on Infinite Earths was a massive DC multiverse crossover that attempted to pare down the 837,000 alternate Earths (some populated by the superheroes DC Comics had acquired by buying out other comic book companies over the course of 50 years, others created just to resolve DC's own legendary Continuity Snarls) into one world, obliterating many "Alternate Earth" characters in the process.
Fables is about various figures from fairy tales and folklore living secretly in a neighborhood in NYC.
Castle Waiting is another multiplayer fairy tale crossover, featuring characters from many different tales living together in a castle.
The Indelible Alison Bechdel offered a mash-up of various lesbian and gay comic artists, who threw their characters into the same world for a party. As the mash-up included Diane DiMassa, creator of Hothead Paisan: Homicidal Lesbian Terrorist, the results were hilarious.
A non-canon Judge Dredd story in the 1980 Dan Dare annual had Tharg bring all the popular characters currently being published in 2000 AD as well as the Starlord to Dredd's apartment for a surprise party. Then the robots that actually write the comics went on strike, forcing the characters to beat them all up.
The Alan Moore comic Albion shoves together a whole bunch of British comic characters of varying obsurity, most of whom are imprisoned by the Government as part of The Masquerade.
Deadpool Killustrated is similar, except that the Merc With A Mouth is killing all the great characters of fiction, in the conviction that if he destroys the heroic archetypes of literature the Marvel Universe will never exist.
Many Seltzer and Friedberg works, such as Epic Movie and Disaster Movie, could be considered Massive Multiplayer Crossovers, insofar as they feature many characters and plot elements (or weak parodies thereof) from recent movies and mash them all together. By all rights, this really should produce something worth watching on some level.
A version of this in Star Trek: Generations, which featured the captains of two Enterprises from completely different eras (Kirk and Picard) in the same film. Star Trek has done this numerous times, if you consider the different series separate entities of the same intellectual property.
Van Helsing, which features the eponymous monster hunter battling Dracula, a werewolf, Frankenstein's Monster, Igor and Mr. Hyde.
The Argonautica (more commonly known by the adaptation Jason and the Argonauts) by Apollonius of Rhodes (3rd century BCE) is one of the very first Massive Multiplayer Crossovers, arranged in what would become a fairly classic method — basically throwing one or two dozen heroes from various separate Greek myth cycles together on a boat with a common mission. This of course makes the Massive Multiplayer Crossover Older Than Feudalism.
Many of the same characters also appear in the story of the Kalydonian Boar Hunt - which, Depending on the Author, may occur before or after The Argonautica.
According to some religious studies texts, this has also gone on in many, many other myths: the most notable involve various saints meeting each other. This goes on even today.
The MachinimaBeans. Firstly, it crosses over characters from three different series and is made by three different machinima directors, then the storyline involves various internet memes... Oh. And it's made on Super Smash Bros.. Making it a crossover on a crossover.
The BBC Radio 4 series The Rivals is a series of adaptations of various Victorian detective stories by people who weren't Arthur Conan Doyle, based on the short story collections The Rivals of Sherlock Holmes edited by Hugh Green. What makes the radio series a Massive Multiplayer Crossover rather than a sequence of unrelated dramas is that Inspector Lestrade is added to every story as The Watson, placing them all in the same Verse.
Although there are many similarly-themed RPs, Milliways Bar deserves mention just for its size. The basic premise is that any character from any fandom can find themselves at the titular bar... and sometimes it seems like just about everyone has.
Stand-up comedy troupes sometimes feature a series of comedians who usually headline their own shows:
The Original Kings of Comedy
The Blue Collar Comedy Tour
The Comedians of Comedy
The Spelljammer and Planescape settings were designed with this in mind. Spelljammer in particular has rule books dedicated to detailing the Crystal Spheres of settings such as Krynn, Abeir-Toril, and Oerth. Where as Planescape has portals to every type of world imaginable.
Similarly, the Hero Clix tactical miniature combat game works on a Point Build System basis and includes characters from many comic books (Marvel and DC, but also "indie" ones); Superman, Dhalsim and Dark Phoenix against Dr. Manhattan, Beta Ray Bill and someone else to match character points is a perfectly reasonable battle.
Ani-Mayhem, a Collectible Card Game produced in the middle 1990s by Pioneer Entertainment that set characters from many different anime settings — surprisingly including many series licensed by companies other than Pioneer — in a grand romp across their various worlds.
Seussical has characters from several of Dr. Seuss' books
Ask That Guy: (speaking to the various reviewers present) In fact, I think there's a lot of you who want to do crossovers, aren't there? Because everybody really eats that shit up. Everybody: Yeah!
All the cast stayed in Chicago for a couple more days to film crossovers after this. Highlights include Linkara being force-read his own Massive Multiplayer Crossover fanfiction by the Sage, the Ultimate Warrior writes a comic series so bad that reality breaks down and Linkara and Spoony keep changing into different Alternate Universe selves and the Critic and Nerd joining forces to review... a Making Of of a Teenage Mutant Ninja Turtles Tour. Yeah.
John C. Wright has come up with a campaign setting (further down the page) which revolves around "anything goes", and incorporates roughly a billion various setting and universes into one complex setting rife with secret societies and inter-universal conflicts.
Disney owns all the characters it uses plus a whole network. House of Mouse has Mickey and Co. as hosts of a nightclub/theater, with the characters of the feature films as the audience.
Many classic Hanna-Barbera characters have been used across the board, in shows like Laff-A-Lympics and Yogi's Gang; this is still done, but with a more satirical bent (e.g., Harvey Birdman, Attorney at Law). The largest gathering of characters, however, came with a Hanna-Barbera 50th anniversary special in 1989 as all the characters gather together to pay tribute. This doubles as Roger Rabbit Effect as they interact with Joe and Bill themselves, along with special hosts Tony Danza and Annie Potts.
Nearly all recent Warner Bros. animated television series made references and overt cameos with one another.