Serious Sam has coop in a FPS but IMO it's pretty boring after a short time (kill, die, respawn, repeat). The Rogue Spear series has decent coop mulitplayer but the AI is kinda dumb and makes up for it by cheating.

I believe Swat 4 has coop multiplayer as well but I haven't tried it.
Guild Wars is an excellent MMORPG with great graphics. Unfortunately, the characters are capped at level 20 and the missions get a little bit boring once you're well into the game. Still, considering it's *HUGE* and the online play is free I'd highly recommend it.
I read that the new "Brothers in Arms: Earned in Blood" has coop multiplayer but I haven't tried it either. If anyone has please report back and tell us about it.