Hmm apparently TF2 is Free to play this week. But after that it’ll remain Free to Play (F2P) forever. It’ll be supported by the microtransactions for hats, weapons, etc.
http://kotaku.com/5815052/team-fortress-2-is-now-free-free-forever
Update: News directly from Valve on the F2P TF2.
Meet the Medic!!