+1000 for this thread, I am very interested on those Kind of games, I thought COD4 had 2 player cooperative... and was wrong.
I am getting some of those games on the 360 to play with my friends, Since I have Gears and Halo 3, which other game will you recommend me to get?

Wii - Elebits (4 player co-op)

 It does?  huh.  I didn't even know that.  During the story mode?

motogp 06 => 4 player splitscreen

Ok, I updated the list. It is now in a spreadsheet. I know there are more games... keep them coming and I will keep adding them. Help me out here... Some of you play a LOT of games and know what is co-op. Just take the time to post...

NOTE: I am purposefully leaving out sports games because it is assumed most all will have both competitive and local co-op.

NOTE2: I am leaving out fighting games (except SSBB) unless there are others that have a true co-op campaign.

NOTE 3: LOCAL CO-OP ONLY. There are lots that do co-op over the internet. That isn't for this thread.

Keep adding and I'll keep updating the list occasionally.

If you want to count it, Pixel Junk monsters, strategy, 2 player same screen

Assault Heroes!!!! It's an XBL game (that is, you download it from XBL and it has local coop)

PS3

Blast Factor, 4 player same screen co-op

Pixeljunk Monsters, 2 player same screen co-op

Lego Star Wars for all platforms has local (and online for PS360) platformer game

EDIT you've already got it, you just have not added PS3 in