Hey, how about Team Fortress 2? It's a free to play FPS shooter with different classes (9 of em)!
I also got paid suggestions like:
Counter Strike (1.6/cz/source) - I'm very good at this game - £7 1.6+cz £15 source
Team Fortress Classic - The older team fortress
Garry's Mod - Huh, it's around £6 on steam