It's a new free game available via Steam to anybody who has purchased a source-powered game (i.e. Half-Life 2, HL2: Ep 1 & 2, Portal, CS:S, TF2, etc.). In addition to this, it's extremely fun. There are two teams: zombies and survivors. Generally, most people will join the survivors team. If you're a survivor, you find guns and melee objects to beat down the zombies. Zombies will try to kill the survivors. When a survivor is killed, the player becomes a zombie, adding to the numbers. The game ends when no survivors are left.

I played it for about 2 hours today. Really good stuff. Granted, it's probably nowhere near as good as Left 4 Dead will be when it's released, but it's still a nice source mod.