Comic Books / Massive Multiplayer Crossover

The League of Extraordinary Gentlemen has a Super Hero team composed of multiple public domain characters.
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.
Planetary is about an investigative super-team in the Wild Storm universe. Their members have had run-ins with Dracula, Sherlock Holmes, Jenny Sparks from The Authority and multiple versions of Batman.
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.
Joe the Barbarian does this in a manner similar to The Indian In the Cupboard.
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.
Shi / Cyblade: The Battle for Independents featured many independent comic book characters, including Cerebus, Bone, Hellboy, Madman, Megaton Man, Scud the Disposable Assassin, The Tick and Usagi Yojimbo. Many of the same characters have also appeared in the normalman/Megaton Man special, Gen¹³ ABC and War Of The Independents.
Freddy vs. Jason vs. Ash, and its sequel Freddy vs. Jason vs. Ash: The Nightmare Warriors.
Dark Horse Comics had Alien versus Predator versus Terminator, Superman and Batman versus Aliens and Predator, and two miniseries that crossed over Witchblade, Aliens, The Darkness and Predator''.
Fantastic Four: True Story has the FF traveling to the realm of fiction, that is attacked by Nightmare, lord of the dreamworld. To fight him they join forces with Dante Alighieri, the Dashwood sisters, Faust, almost all William Shakespeare's protagonists, Frankenstein's Monster, Robin Hood, the Ivanhoe cast and others, while Nightmare gains allies in the Sheriff of Nottingham, Dracula and Long John Silver. Many other characters like Tarzan make cameos and in the end Johnny summons lawyer friendly versions of James Bond, Megatron, Steven Seagal and Chuck Norris to defeat Nightmare's army.
- 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.