Wolfenstein Enemy Territory is a FPS that isnt terribly demanding,
OK, thanks. I am downloading it right now.
Any more ideas?
Make sure you patch it to the latest version(even though the latest version is still a few years old, 2.60b i think), Lots of custom maps/addons on it(noquarter is my favorite mod).
Heres a list of a bunch of open source games:
http://en.wikipedia.org/wiki/List_of_open-source_video_games
You can try my 'mm'orpg, but there isn't really anyone who plays it right now.
http://www.mirage-realms.com/main/community/viewtopic.php?f=32&t=290