Zombie Panic Source is a third party mod designed around the Half Life 2 engine for Steam. This is a player driven game where you join a server, select either to be a human or zombie, and try to survive! The format of play is Counter Strike-esk round by round. All players start in a lobby, and select the team they want. The game has two modes of play; Survival, where humans must bludgeon, decapitate and blow the zombies into little pieces, until the zombie respawn counter is 0 and all zombies are dead, or objective mode where the zombies have unlimited respawns and humans must complete tasks built into the map to win. This game is a free download from the Zombie Panic website and worth having a look if you have any source engine game installed (Counter Strike Source, Half Life 2, etc).