I've been addicted to this game for several months now. It's a HL2 mod if the source in the title didn't give that away. Essentially you start as a survivor, try to find some weapons and a safe place to set up camp. You can build barricades by throwing furniture into the various entrances and try to stay alive for 10 minutes. If the main zombie gets a hold of you there is a chance you can get infected and will eventually turn into a zombie. If you die then you come back as a zombie and get to go after the people that were supposed to be watching your back. You can get it free online from steam if you own any source game.