My son is playing the Left4Dead demo and it looks pretty cool. In a nutshell, it's a 4-player co-op game where you take one of four player roles. Each player is somewhat different in weapons and skill set. Your goal is to get out of the level alive while being trapped by hordes of brain hungry zombies. Some of the zombies are the slow moving variety while others are the fast ones. It's fast action, team based combat.
I'll be picking this one up if for nothing else than the opportunity to kill hordes of zombies with shotguns and baseball bats.
"Grab a club or a torch. Beat 'em or burn 'em. They go up pretty easy."