Go to this link, select the game, go to the checkout and get the game and then add the key to your steam library.

https://www.humblebundle.com/store/p/stealthinc2_agameofclones_freegame

It says it expires (goes back to full price) on the 31st August, it does not say what time.

As per http://steamproxy.net/games/L4D2/announcements/detail/1964991833269216480

"To celebrate the holidays in a special way this year, Left 4 Dead 2 will be free until 10AM PST 12/26"

You can get the game here: http://steamproxy.net/steamstore/app/550

This is worth doing even if you don't play it any time soon.

Merry Christmas.

Check here: http://imgur.com/a/g0UeC#0