When I first read about Nexuiz, I decided to give it a try, as I was for a while a Half-Life addict.

I downloaded the package 161MB package. The zip file just needs to be extracted and after clicking the windows executable I could choose a server from the list and the fun started.
Speed and graphics are great. Within a couple of minutes I was alright on the keyboard and mouse controls and landed my first 'kills'. The worlds are full with weapons and things like elevators or trampolines. The current package includes 17 maps, 28 playable characters, and 10 music tracks.
The Nexuiz First Person Shooter game is fun and best of all it is free. It even comes with source code as the whole game is released under GPL.

Nexuiz is built on the Darkplaces engine, which is a modified version of the original Quake.
Darkplaces features realtime lighting and stencil shadows, bumpmapping, gloss, bloom, and totally rewritten network code that supports up to 64 players on a single server.
It runs on Windows and on Linux. An Apple version is supposed to come soon.
More details and free download on the Nexuiz site.