Nexuiz

Nexuiz, a "simple fast, intense and completely free" game.
Nexuiz has excellent graphics, although these graphics are full on, you can still get rather smooth play these graphics are also of very high quality for the menus and interface, it is very well designed and fairly easy to navigate. They come together nicely in nexuiz 2.5.

All of the nexuiz game types are fairly simple to understand and rather intense, there physics enable you to zoom around the map while fragging players.

This game isn't just limited to one operating system, nexuiz is a cross platform game that can be played on windows, linux and mac. To top it off it is open source and players are always welcome to submit ideas or patches. This contributes to their community, where most people are friendly to each other.

The only down side to nexuiz I can find is that when using ultimate graphics it can sometimes lead to low fps, but all the graphic setting able to be changed to suit your computer.

Nexuiz is truly setting the bar for open source gaming with 2.5 - 10 for nexuiz.