Since the release of the game, Valve has occasionally made it free for short spurts of time, but now it's free for good.
In a recent interview with Develop-Online, Robin Walker of Valve revealed that Team Fortress 2 will now be a free-to-play title. According to Walker, for almost a year, the team has been toying with the idea. Data from periods where the game was made free for a limited time convinced Valve that " TF2 would be more successful as a completely free product.”