Nexuiz runs on an improved Quake engine called DarkPlaces. The engine has been undergoing on-and-off development by Lord Havoc, the game's creator, for several years. On icculus.org, Lord Havoc says he developed a custom OpenGL-only engine for DarkPlaces, and other modifications that "support Windows WGL and Linux GLX and have greatly improved graphics and image quality."
I grabbed the latest version of Nexuiz from the Nexuiz project page. It's licensed under the GNU GPL -- version 2.0 or later, your choice -- with versions available for the Windows, Mac, and Linux platforms. The tarball includes binary versions for each platform as well as the source code. Linux executables are available for both 686 and 64-bit architectures. Each has two client and two server versions: one set is built using the Simple DirectMedia Layer (SDL) graphics library and the other using GLX. I played both the SDL and the GLX 686 versions on Ubuntu 7.10 and couldn't tell the difference between them.
http://www.linuxlinks.com/portal/news/article.php?story=20080513135412992