L4D ... L4D2 are fun for co-op shooting. License are pretty cheap... Can get for 5 to 10 bucks. Sometimes less on sale...
I just built another system for playing older games to do co-ops myself. Of course there is always the tried and true (unreal) GOTY ... Can have as many players and bots as you want with unlimited maps. There are a few good L4D custom maps that are pretty good.
I've heard that FC4 is pretty good, where you can drop in and play with a co-op on there engine. I have not installed it yet, but it's on my radar for the next go around. I liked FC4, so it should be fun ... Would be cool to get two buzzer helicopters flying around trying to shoot each other down.
Speaking for license on steam... anyone know if you can just get one license and share it to play co-op? Or are all buying 2 license required (always)? I've been going the double license route. but... buying 2 license can get expensive for mainstream games just to play co-op once in awhile.