Pretty sure Left 4 Dead 2 is the next free-to-play for Valve and this is an initial test. They are probably adding items and upgrades soon, but they do it very

Pretty sure Left 4 Dead 2 is the next free-to-play for Valve and this is an initial test. They are probably adding items and upgrades soon, but they do it very tastefully like in TF2 and CS. They probably have a growth curve that starts initially as a pay game, then on taper, open it up, then add in items/content and the markets. Very smart and they have done them one at a time. The markets and goods actually make the game fun to go back to and don't bend game balancing too much, but add a layer of fun. They are pretty much the model for good in-app purchases on desktop.