Lego Star Wars 1 and 2. Great fun. Together they cover all the Episodes, try it out. GLTron is fun for some mindless lightcycle racing too
I didn't know brainbread was co-op, i thought it was mostly just killing zombies and getting killed by zombie players.
System Shock 2 (you have to d/l a multiplayer mod for it to work though). Played for 48 hours straight with my friend, and still didn't complete it. Actually I think we got about 30% of the way through.
Doom (oldskool doom, not that Doom 3 crap) is pretty fun co-op... but you have to play on ultra violence or nightmare for it to be a challenge at all. And if the graphics aren't good enough for you, get Doomsday and the jDoom Resource Pack (If you need help getting it to work, go here).
hey Dee, i just downloaded brainbread a few days ago for my zombie fix. maybe ill see you around other then L4D (so close to march release), the only one i can think of is Rainbow Six: Vegas.
EMPIRESMOD! lolohwait I've been mostly playing online DS games, but if I recall, you don't have a DS, so...
I've got Animal Crossing: Wild World, but I'm paranoid about letting people into my town. I do have Metroid, too, though.