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.