As a sci-fi fan, I spend approximately 50% of my time thinking about what I would do in the case of zombie invasion. (You have to avoid hospitals, they'll be disease ridden, and you'll be better off in small groups where you can observe the behaviour of your friends. No point fortifying your base if you'll get bitten in the back) Anyway, back to the point.
Set in the immediate aftermath of a zombifying virus, Left 4 Dead is a non-story driven team based FPS. Not to be confused with other 'team' games, in Left 4 Dead, if you stray from the group, you die. Requiring tight-knit teams to get past the toughest difficulties, there is a surprisingly long amount of gameplay time in the 4 chapters, each of which is composed of 5 levels. Increasing in difficulty, the final level of each chapter is a 'hold the fort' style game, in which you must defend yourself from the zombie horde until rescue arrives.
On top of this, there is the superb versus mode, in which two teams alternate between playing as the four human survivors and the special undead. The special undead feature in the regular game, but in versus they form deadly teams if the players work together. The hunter, the most common, is fast, and can leap onto you, incapacitating you until a friend can shove or shoot them off of you. The Smoker can wrap its ludicrously long tongue around you, hauling you towards it. Defenceless, you must wait for a team member to shoot the Smoker, or untangle you from it's tongue. The boomer is a rather rotund fellow, who will vomit on you and your team. Sadly, this vomit draws the undead, bringing the horde screaming towards you. And don't shoot it too close up either, it'll explode with much the same effect. Finally there is the tank, who only spawns once or twice a level. You'll see him on the game trailer, the big one. You'll see what I mean.
If you're looking for both great teamplay and zombie shooting action, this is teh game for you. Go on, treat yourself