According to the Left 4 Dead Announcement Page the game will be free until 10AM PST December 26th. To claim the free copy of Left 4 Dead 2 all you need to do is install it. The great thing is once you have installed it before the deadline the game is yours to keep forever.
So what are you waiting for? Head over to the Left 4 Dead 2 Steam Store page and download it! Did you get it? Let us know in the comments!