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.