Lego Star Wars is fun, and Ghost Recon 2 was alot of fun co-op. Gears of War is really good as well.
I think the Blazing Angels games offered co-op play, but I never played them. Crackdown offered it and while I enjoyed the game I never got the chance to team up with anyone.
I'll second the Ghost Recon and especially Gears of War.
Crackdown is only co-op if you have 2 systems, 2 copies of the game and 2 tvs. So it's probably useless for your situation.