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 http://www.urbanterror.info/

2. open source games
- Open Arena http://www.openarena.ws/
- Assault Cube http://assault.cubers.net/
- Smokin Guns http://forum.smokin-guns.org/
- Xonotic http://www.xonotic.org/ (Unreal fans should feel like home playing this)
- Red Eclipse http://www.redeclipse.net/
- 0.A.D http://play0ad.com/
- Warzone 2100 http://wz2100.net/
- Battle for Wesnoth http://www.wesnoth.org/

If you want more games like this start your search from here http://www.lgdb.org/category/source/open