Today only, Dec 26, 2013, till 1 pm est, Valve is offering Left 4 Dead 2 for PC, Mac, and Linux for FREE.

Set in the zombie apocalypse, Left 4 Dead 2 (L4D2) is the highly anticipated sequel to the award-winning Left 4 Dead, the #1 co-op game of 2008. This co-operative action horror FPS takes you and your friends through the cities, swamps and cemeteries of the Deep South, from Savannah to New Orleans across five expansive campaigns.