From 3D Scrabble to Command and Conquer this list should help you find something fun to play.
Red Ferret has put together a three part series showcasing 70 downloadable free and open source games. They've added great descriptions to all the games. A few games are free while in pre-release, have a free demo, or have a java script version. There aren't many of them and all are noted in the descriptions. All the games run on Windows. Some are cross platform, so if you are running Linux or OSX check the game site to see what platforms are supported.
- 70 of the Best Free and Open Source Games for Download (Part 1 of 3)
- 70 of the Best Free and Open Source Games for Download (Part 2 of 3)
- 70 of the Best Free and Open Source Games for Download (Part 3 of 3)
At the time of this writing all the links were working and went where they said they would.
There's not much else I can add other than have fun playing. :)