Beside commercial games there are some few games free and/or open sourced at the same time. They cover areas like FPS and strategy. Here are some suggestions:

1. close source games:
- Urban Terror

2. open source games
- Open Arena
- Assault Cube
- Smokin Guns
- Xonotic (Unreal fans should feel like home playing this)
- Red Eclipse
- 0.A.D
- Warzone 2100
- Battle for Wesnoth

If you want more games like this start your search from here