Steam Store link
Try this link guys..
Well if you copy that link and paste it into your browser (I'm using chrome at the moment) then it should open the steam install dialog.
Best Answer +Frank B. , 25 December 2013 - 19:17
Left 4 Dead 2 - FREE Today Only!
Left 4 Dead 2 is FREE today only on Steam!*
To celebrate the holidays in a special way this year, Left 4 Dead 2 will be free until 10AM PST on 12/26.
It will be free as in, grab it now, pay no money, and it is yours to keep forever – FOR FREE!
To claim your L4D2 gift, all you need to do is install it.
If you don’t install, it goes back to its regular price and you miss out! The zombie apocalypse is more fun with friends so make sure to tell all your mates to install it as well.
*Offer ends December 26th at 10AM Pacific Time.
Source: Steam NewsGo to the full post
Edited by Roger H., 26 December 2013 - 02:17. Reason: Added direct install link - Edited content