your mother on 03 February 2012
How about Killing Floor, Brink or Bad Company 2: Vietnam? They are all FPSs, and can be had on the cheap.
Wait, what was I thinking? if you are into hardcore tactical co-op you owe it to yourself and your friends to try out SWAT 4. It's an old game but is awesome in co-op among buddies, and requires real teamwork on the level of Tom Clancy's Splinter Cell: Chaos Theory's co-op mode and is just as fun.
Check these out:
http://www.youtube.com/watch?v=Nq6Tsv9M9ns
http://www.youtube.com/watch?v=foRXx7WlWWA
