This is a mod For Gmod10 + CS:S.
If you own these games (or at least Gmod, though CS:S makes you have a lot more wep and props models) go to the ReDead forum, right now. Download the info, put it in your folders, and play in any of the servers people put up.
If you like zombies, you will just LOVE this game mode.
Its basically the upcomming game Left4Dead, except its been released for a while now, and its amazing!