Bulletstorm had co-op as well I beleive....I loved that game.
Fantastic game, but the co-op is only for the competetive score attack mode, not for the campaign. Which is a shame, because the story in that game was awesome, in a pulpish kind of way.
Gears of War, as some have mentioned, is really good, and they just get better as the series progresses.
Someone mentioned Halo
, but I'm not sure why Halo Reach
wasn't mentioned. It's easily the best game of the series, even if it lacks some of the things that made the other games good.
Splinter Cell: Conviction has a pretty good co-op part, which is like a separate story from the main one.
F. E. A. R. 3, while not the best entry in that series ( certainly better than FEAR 2, but nowhere near as good as the first game ) I've never played in co-op, but the game is basically built around it, so it could be worth checking out if you're into horror, blood and gore. It also has a game mode titled F**KING RUN!!!
which sounds pretty cool. I mean, come on...