There is no catch! Team Fortress 2, arguably the best class based multiplayer game is now free to play. This comes as a part of the Valve’s most ambitious update, the Uber Update for Team Fortress 2.
Team Fortress 2 will follow the free to play business model and set the stage for other games. It will be financed by the sale of in-game store items which are already popular.
If you are one of many, who already purchased the game, you can read your benefits in this FAQ which compares Team Fortress 2 free and premium accounts.
Don’t forget to check out Team Fortress 2 Crashes, Errors troubleshooting guide in case you experience any of these.