Team Fortress 2

Team Fortress 2 is Free to play, Multiple player online FPS. You can download it free steam.

http://store.steampowered.com/app/440/

Download size is about 6 GB. This is one of the best free games. There are large community and market arround Team Fortress 2. You get items drop when playing game, that can be sold to make real life money, buy paid games etc..