There are lots of great co-op games for PC (some of which also cross over to platform).
Left 4 Dead has already been mentioned, 1 and 2 and both good, but 2 is harder than 1.
Most of the Battlefield series is fun to play co-op against bots. Avoid BF:Vietnam or BF:2142 though. The others are good. Even ol' BF1942. I still play it from time to time, especially with the Desert Combat mod.
The Serious Sam series is an absolute riot to play co-op.
You can call the Unreal Tournament series co-op, as you can put both humans on one side and fill the other side with bots.
Call of Duty: World at War has a co-op campaign, and it's not bad. It suffers from "the world is brown and bloom" though.
Borderlands. Borderlands borderlands BORDERLANDS.
Neverwinter Nights series... for that matter, Baldur's Gate series, though the multiplayer isn't as good in BG1. NWN1 was better than NWN2.
All of the above I play/played with the little woman.
Technically, most RTSes these days can be "co-op" because they have maps that can support up to 8 or more players, and you can set teams. So it can be humans vs AI. The best of the genre currently is the Supreme Commander series, though the Dawn of War series is also a lot of fun. Demigod is also good, and it's a lot shorter and less involved.
Other games I know of that support co-op (but I never tried):
Saints Row 2
Marvel Ultimate Alliance series
Zeno Clash
If you are interested in info about co-op games, try
http://www.co-optimus.com