Good co-op PC games?

I've been looking for something from the FPS or RPG genres, haven't found much that supports campaign co-op.. Is this more of a console thing?Good co-op PC games?
fps? only operation flashpoint comes to my mind... coop games are usually action type... gears of war, full spectrum warrior (but u got only 1 team), chaos theory, kane and lynch (I didn't play this one, someone told me it sucks), or if you don't mind campaign, anything against AI in regular MP (but none of the games since 98 have actually AI which couldn't been beat by a 5year old cs player with finger in nose, so this option sucks too) I have no idea why developers are unable to add coop to a game.. it should be standard for a modern game to include mp, sp and coopGood co-op PC games?
Left 4 Dead has excellent coop. :)
hey I almost forget... there was mod for HL2 called synergy in development... althought when I tried it year ago.. I loaded into a map with a buggy and those starhip trooper things on the beach with 5 other player.. one of them jump into a buggy and drove over those 3 guys and then crashed so I shot him and that was the end ;) But it was a year ago so maybe it has bugs fixed and there will be more players.. definetely worth try...and left 4 dead is highly prised, but I bought it for my friend for Xmass he played it like mad and throw it away and told me that you need to know those ppl to enjoy it..
L4D hands down if you want an FPS co-op game.For RTS games, I enjoy playing CoH with friends, but there is no campaign co-op just skirmishes against other players or the AI. Red Alert 3 has campaign co-op, although I haven't been able to try it out yet.
FPS: Serious Sam series, Ghost Recon Advanced Warfighter 1/2, Rainbow Six Vegas 1/2, SWAT 4, Vietcong 1/2 and Call of Duty World at War. No one lives forever 2 has some great coop missions too. I also like to suggest the Killing Floor mod for Unreal Tournament 2004. There are also great coop mods for Doom 3 and Far Cry, but I have never tried them.
Swat 4's co-op is excellent, even better than Left 4 Dead's imo.
RPG:

• Neverwinter Nights + Hordes of the Underdark
• Neverwinter Nights 2 + both expansions
• Titan Quest Gold
• Diablo 2 + Lord of Destruction
• Dungeon Siege 2 + expansion
• Marvel: Ultimate Alliance
• Freedom Force
• Freedom Force vs. the Third Reich
• Sacred Gold

FPS (that hasn't been mentioned yet):

• System Shock 2

If you're a fan of realism, Armed Assault offers cooperative campaign play online with a community-made island map. The objective is to move through AI-controlled cities by taking-out AA guns, armor, and infantry forces. It's a lot of fun and can be very engrossing if you play in a squad with real players.
Might want to check out Borderlands but that game isn't out yet. Out now l4dead is pretty much it. Hellgate was good for that but its gone now.
A couple buddies and I enjoyed playing Rainbow Six Vegas 1%262 in the online coop. Left 4 Dead as previously mentioned is also always fun.
No One Lives Forever 2.
Left 4 dead and Swat 4
Left 4 Dead is all about co-op, and is one of the best games valve have done (that says a lot)
It was mentioned before, but Synergy, a mod for HL2 allows you to play the game co-op. There are still a few glitches, but its very playable.

blackheads scars