You want L4D Free 2 play ? Join that group !!
Actually the game is pretty dead, so we need players, maybe to make some cups and other thing. The best way to get players is to make the game free to play.
So if you want L4D free to play, join the group and invite your friend !
Then we'll contact Valve to try to convince them to make it F2P.

http://steamoss.com/groups/L4DF2P