Turtle Rock Studios, the co-developers of Counter-Strike: Source,  has created this ambitious co-op zombie game.  Running on the Source engine, Left 4 Dead leaves four armed people in the midst of a zombie outbreak.  They must work together through each stage to make their way out to escape the outbreak.  This game was designed for co-op play, so players will have to help each other out by sharing ammo and knocking the zombies off of each other.  If you are a fan of zombies, then this game sounds llike a must!