Left 4 Dead is a first person shooter for the PC/Xbox...about motherfucking zombies.
You play as a team of four survivors (on your own, or with four friends, which is DEFINITELY the way to do it) and fight your way to safety. The zombies are fast and merciless and capable of some crazy mutant feats, but the AI is where this game gets really crazy: rather than have fixed re-spawn points for enemies and locations for items, the game figures out your playing style and the zombies and items are arranged accordingly, making no two playthroughs the same.