Erm, Red Alert 3 Co-op is pretty fun, but I'd say Rainbow Six Vegas 2 is pretty darn good. Speaking from a console perspective.
Unreal 2004 fucks you all in the mouth sack but co-op I say Garrysmod ---------- Post added at 06:11 AM ---------- Previous post was at 04:19 AM ---------- I guess L4D is fine, as long as we all remember what E equals.
Unreal 2004 isn't coop. Well, my friend did make a coop mod for it with everything from Unreal including maps, monsters and weapons but aside from that its not coop.
I heard Saints Row 2 Co-op was awful, but I haven't played it. The Online Multiplayer is horrendous though.