Rob Williams of TechGage has written an article on the top 10 free linux games. The criteria are as follows:

So how does a game make this top 10 list? Well, from 10 - 1, they have all been listed according to how many votes the game received. Though there were around 50 games total that were brought up in forum posts, most only had a single vote. Even our #10 had only four votes, so it's obvious that the top games are there for a reason. In order to qualify for the list, the game had to meet these requirements:

Has to be 100% free... no strings attached.
Cannot require wine to run.
Open Source not a requirement.
Installation must be painless.
Cannot be an add-on to a commercial game.

10 - KMahjong
9 - America's Army
8 - Armagetron Advanced
7 - Cube
6 - Pingus
5 - Neverball
4 - Nexuiz
3 - Enemy Territory
2 - Frozen Bubble
1 - Battle of Westnoth

Wow! I only play Frozen Bubble! [I rarely play games anyway]