I'm usually more of a mmorpg fan, but since the games I'm waiting for won't be out for some time, I just felt like playing some FPS to fill up my spare time.
I don't mind if it's not free, I'm just looking for some FPS games that have a good co-op mode in it.
Theres a half life mod called sven co op. It lets you play some custom missions cooperatively with people online. Pretty fun actually. Also try out garrys mod.