There is no catch, and the title is 100% correct. Team Fortress 2 is now free to play! Team Fortress 2 has had over 200 updates in the last 4 years, and the Über update that has just been released is the largest content pack delivered to Team Fortress 2 since it shipped in 2007 (and the updates will keep coming).
Team Fortress 2 Free to Play is available for PC and Mac players via Steam, so what are you waiting for? Get out of here and start playing.
For more information on Team Fortress 2 Free to Play, visit http://www.teamfortress.com/freetoplay.