LittleBigPlanet
Motorstorm has 4-player splitscreen, and really shines when you can watch your friends' ragdoll bodies fly out over the cliff you just pushed them off of.
Haze -- haven't played it, apparently 4-player co-op
Marvel:Ultimate Alliance (for Champions of Norrath-style action RPG fans, this is the only 4-player local option this gen so far. Wait -- there's also the Fantastic Four game, which looks based on this one. Haven't tried it.)
Downloadable games:
Comet Crash: The most unadvertised game on PSN, and the second highest rated of all time. A mixture of tower defense and RTS, and you also fly around geometry wars style and try not to get shot. Actually a deep game.
pixeljunk: racers (I think this goes up to 7 players, don't actually own it.)
pixeljunk: eden (this is actually 3 player, but pretty awesome in co-op.)
Rag Doll Kung Fu: A fun Smash Brothers-ish ripoff with sixaxis controls that are surprisingly good, but still get old. Got to be fun as a four-player game, though I've never tried it with more than 2. (Keep forgetting to load it up when people come over...)
Noby Noby Boy: If you're like most people, you will play this game for 1-2 hours and then never again. But, it only costs five bucks and is good fun... hmm, make that hilarious chaos. Just make sure the person controlling the camera in multiplayer knows what they're doing, otherwise they're going to get yelled at A LOT.
Coming out in June (hopefully?):
Trine -- this looks like a pretty awesome side-scrolling puzzle/platformer with nice looking fantasy-themed graphics. Only 3 players. I love games where you work together to solve puzzles (see the classic PS2/DS game, Cookie & Cream.)
Also, sports games are generally 4 player, as are Rock Band and Guitar Hero.
*edit*
missed the Xbox games being in-bounds as well.
CASTLE CRASHERS -- the only game that competes with LBP for four-player co-op. Think Teenage Mutant Ninja Turtles, Battletoads, Double Dragon -- it's like those games, but with awesome art that looks hand-drawn for a comic book about knights.
