Valve. The Cold Stream update, which is already available on PC and Mac over Steam for free, will cost 560 Microsoft Points ($7) on the console.
Cold Stream includes the fan-made campaign of the same name, along with the four original Left 4 Dead maps: Crash Course, Death Toll, Dead Air and Blood Harvest. Friday night we'll see if the new pack gets people "RELOADING!" the three year old game.