Left 4 Dead is pretty awesome.
There was a zombie mod for COD4 which was cool too, but this is just a very well finished game. Plus you can play as the zombies, and boss zombies too. I've been playing the demo and it's pretty fun.
IMO, the CS mods have always been better than CS. TF2, and L4D are pretty great online games you can just pick up and play. Personally, I've always liked Soldier of Fortune 2, and the COD series more for FPS, but that's cause they have better character physics/control. Although, it's really hard to beat the team play of TF2 and L4D.
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