I am not sure when this "sale" will end but Left 4 Dead 2 is currently free as Valve's gift to us. Simply put, if you have not yet purchased this game, you owe it to yourself to take the few minutes and claim a copy. That is, take the few minutes several times until Steam actually loads because the service is currently getting hammered. I guess a lot of people got Steam Gift Cards for the holidays?
Yes, you are not the only free game.
We, here at PC Perspective, have played Left 4 Dead 2 a couple of times after the weekly podcast when we did our gaming nights. It is definitely a fun game especially when you enter Versus Mode. Even though it will probably take several tries to get the Store page loaded, I have successfully gotten it loaded once or twice. Good luck, Happy Holidays, and beat a tank over the head with a guitar.
I am not sure if you need to do this through the Steam Client (because I already own the game) but you can try the website in your web browser here.