The Steam users can play Left 4 Dead 2 from 24 to 28 September without any charge.
How to play Left 4 Dead 2 for free
To play Left 4 Dead 2, visit this game store page on Steam and press "Play Game" button.
Left 4 Dead 2 - The Last Stand Update
The Last Stand is a community created update. It was was released today, as the official update for Left 4 Dead 2 game. The Last Stand update includes a new campaign, new survival maps, scavenge arenas, meele weapons, new achievements to unlock and more!
You need to make a purchase in order to be able to play Left 4 Dead 2 after free to play period of time. Enjoy!