Namco X Capcom was a strategy RPG for the PS2 in Japan. It was a crossover between, would you guess, Namco and Capcom.
This is the opening video from the game, which has a nice little song (Brave New World, which, in the later games they appear in, became the theme song of the original characters Reiji and Xiaomu) and some of the most famous video game characters around (and some obscure ones too; Wonder Momo, Bravoman, and the characters from Forgotten Worlds?).
(uploaded by bass877)
While Namco X Capcom never got localized, it’s sort-of-sequel, Project X Zone for the 3DS, which adds Sega into the mix too, did. It’s pretty good if you like SRPGs and are familiar with a fair number of the characters in it.
There’s one more shared universe I want to talk about before getting into multiverses. This one’s a little more complicated. I’m using Namco x Capcom’s status (confirmed canon for Endless Frontier which is considered canon by SRWOG) as a precedent for assuming that Project X Zone is probably canon. And while it involves different universes, it also establishes a few games to be in the same universe. (Note that a fair number of these are connected by other crossovers too, so it only establishes a few new connections.)
Project X Zone seems to establish that Street Fighter, Rival Schools, Resident Evil, Tekken, .hack, Virtua Fighter, Fighting Vipers, House of the Dead, and Dynamite Cop all take place in the same time period in the same world (which is also home to the original characters from NxC and PXZ), which is also the human world from Devil May Cry and Darkstalkers, which also share the same demon world as each other and Ghosts ‘n’ Goblins. It’s also established that Mega Man and Xenosaga are both in the future of this world.
And now for some other games: of course Final Fight has long been established to share a universe with Street Fighter, Virtua Fighter characters appear in Dead or Alive (which shares a universe with Ninja Gaiden), and Soul Calibur is the past of Tekken.
And Lili and Sebastian from Tekken appears in Digimon World: ReDigitize.
There’s no real evidence against Capcom vs. SNK being canon either, so we’ve got The King of Fighters and the other games that it was already a crossover between to begin with.