Team Fortress 2 on Steam

http://www.teamfortress.com/freetoplay/

“The most fun you can have online” - PC Gamer
Is now FREE!
There’s no catch! Play as much as you want, as long as you like!
The world’s #1 war-themed hat simulator!
One of the most popular online action games of all time, Team Fortress 2 delivers constant free updates—new game modes, maps, equipment and, most importantly, hats. Nine distinct classes provide a broad range of tactical abilities and personalities, and lend themselves to a variety of player skills.