- There have been at least two webcomic Massive Multiplayer Crossovers: The Framed!!! Great Escape and the recently ended Crossover Wars
- Though it never crossed over with both at the same time, It's Walky!! had crossovers with both Melonpool and Fans!! that established both as part of its universe (or multiverse in the latter case) and which ended up having a lasting impact on its storyline.
- Kevin & Kell and General Protection Fault had an arc where two GPF characters wandered into K&K's Domain world. The artists managed to work the events into both their storylines that were running at the time, and apparently drew a strip each day—both had two-strip days where the storyline advanced.
- Starslip Crisis: Alterverse War is a mashup of a large number of science fiction webcomics, and had just started as of October 2007.
- Deviant Universe is a comic project involving a huge number of original superheroes and villains from DeviantArt being part of a large superhero universe.
- The webcomics Queen of Wands and Something*Positive had a couple of crossover stories. Later, after QOW ended its run, Kestrel showed up as a semi-regular on S*P.
- Choo-Choo Bear hired the Pet Professional from the webcomic of the same name to kill his homicidal cousin Twitchy-Hug.
- And recently Davan from S*P has been seen texting with Girls with Slingshots after running into her at the wedding of Jameson and Maureen.
- Choo-Choo Bear later had kittens with Sprinkles of Girls with Slingshots, one of whom Davan gave to Roz of Shortpacked!, and another is by Hazel to her cousin Robyn from All New Issues.
- Questionable Content has crossed over with Diesel Sweeties, which has itself crossed over with Scary Go Round. Questionable Content has also crossed over with Applegeeks, which, having crossed over with Ctrl+Alt+Del, would turn the whole group into one quasi-incestuous ball of webcomics. I'm reasonably certain that PvP has been referenced in at least one of these, probably Diesel Sweeties, and since the former has crossed over with Penny Arcade more than once, it's completely cross-eyed insane over there.
- And the Author of Ctrl+Alt+Del exists as a person in the Penny Arcade universe, so that means, if you follow the little red thread of universal strings, that he exists in the same plane of existence as his characters..
- Don't forget that Applegeeks had a major crossover (well, on its end) with MegaTokyo at one point. Which chains the whole thing to a specific date, making things even crazier.
- It gets better. Questionable Content is now having background extras as characters from other comics like Girls With Slingshots, Octopus Pie, Anders Loves Maria and Overcompensating. Since Overcompensating features the author and most people known in the webcomic industry, it means all these characters now exist in the same universe as their creators.
- The cast of Shortpacked! has visited the Coffee of Doom coffee house from QC on occasion, including once when both artists did a take on the same scene. Combine that with the S*P Intercontinuity Crossover Nexus mentioned above, along with all their respective crossovers, and we have a massive webcomic universe that could give Marvel a run for its money.
- Oh, and, Shortpacked! has crossed with Multiplex.
- Sluggy Freelance hosted one of these during the Filler Arc "Sluggy Freelance: Where Are You?" When all of Sluggy's actors are kidnapped (a.k.a. Pete Abrams decides to take some time off) characters from several other webcomics are hired to fill in for them and/or find the kidnappers.
- Several members of the "Webcomics Inc." social networking site collaborated on a crossover comic, where they made all their characters teen-aged and put them into high school together. The result, "WCI High", merged characters from "Good Times", "Mikey's Life", "Everyday Heroes", and "Big Sandy Gilmore". (Of these, only Everyday Heroes seems to be still actively updating.)
- Least I Could Do recently finished the Ultimate Final Civil War Invasion Crisis Thing, which featured the gaming webcomic characters trying to kill the slice-of-life comic characters. (Cyanide & Happiness, meet xkcd.)
- Ménage à 3 and Sore Thumbs crossed over at some point and are implied to be in the same universe, and Ménage à 3 crossed over with School Bites. If this keeps up we'll wind up with every webcomic crossing over to one another.
- The writer of No Rest for the Wicked footnotes her comics so you can get all the fairy tales she's using characters from.
- Final Blasphemy has characters from Mega Man, Final Fantasy, Mario, and several other gaming universes.
- Troops Of Doom has G.I. Joe/Cobra and Star Wars Empire characters, as well as other random characters and an alien race called Legonians.
- The Lady Spectra & Sparky storyline "Demon War" featured guest-stars from several different 1990s-era mini-comics, including Mister Mid-Nite, Doc Saphire, Cassiopeia the Witch, and Madame Boogala.
- AGENCY, though updating at a trickle, often varying from a week to a month between new issues, is still very much active. The comic features select characters from several different franchises such as The Pink Panther, Star Fox, Animal Crossing, SpongeBob SquarePants, Sonic the Hedgehog, and many others in a mystery scenario. It is currently on chapter "Day 3 part 3".
- Gamer9 is set in a world inhabited by all kinds of video game characters.
- And Shine Heaven Now started out as a Hellsing fan comic, but as the series started dealing more with the main plot, it became this. It would take all day to list every intellectual property that got incorporated into the comic (because of all the cameos), so we'll just list the ones with significant importance to the plot: Read or Die, Victorian Romance Emma, Jeeves and Wooster, Revolutionary Girl Utena, Witch Hunter Robin, Good Omens, The Daily Show (and more significantly The Colbert Report which ends up getting a spin-off called The Eagle of Hermes), Little Orphan Annie and Vampire Hunter D. Yeah, those are just the important crossovers.
- Comic Book SNAFU: The first issue alone involves characters from Marvel Comics, DC Comics, Yu-Gi-Oh! 5D's, and Fairy Tail.
Massive Multiplayer Crossover / Webcomics