Now I am definitely not the person to pump up some zombie game, seeing as I do in fact fear zombies, but it is definitely an entertaining game. For those that don't know, this is a game about 4 individuals who are immune to the cliched zombie sickness and have to find a way out of the city while being hunted down by ridiculous hordes of zombies.

These zombies include the normal sprinting zombie horde, a zombie with a long tongue that can pull survivors, a zombie in a hoodie that jumps from roof to roof in order to pounce and maul survivors, the fat boomer that vomits horde-attracting bile, and the hulk-like tank zombie.

The game randomly chooses where to place the zombies, so it won't always be the same run through the maps. Weapons and items are also shifted around, so you have to look for them occasionally (mostly the items).

The games main attraction is the coop, but the versus mode is probably the most fun online. Versus allows you to play with 4 survivors and 4 boss infected (the ones I listed earlier, minus the normal horde). This mode forces the survivors to try to rush through the map to avoid the zombies, but the infected can see them right through the walls (as well as their health status). If you have a good (read: entertaining) it can be very enjoyable.
Of course, as with all Xbox Live games you are bound to encounter the individuals I could only describe with a string of expletives.

Anyway, the game is worth a try, it does use the Counter Strike: Source engine (correct me if I'm wrong) so the graphics aren't entirely pleasant. It is available for the PC and Xbox 360.

http://www.gamespot.com/xbox360/action/left4dead/index.html?tag=result;title;1