Huge news! Valve have just announced that Team Fortress 2 is now a free to play game, permanently. The game will solely be supported by in-game item purchases only. Team Fortress 2 was in the middle of a "free week", and has now become amazingly, absolutely, free!
Valve's Robin Walker said "We've been toying with the idea of making Team Fortress free-to-play ever since the Mann-conomy update, the data we got back from that update leads up to believe that TF2 would be more successful as a completely free product."