ioquake3

This project, ioquake3 (or
ioq3 for short,) aims to build upon id Software's Quake 3 source code
release. The source code was released on August 20, 2005 under the
GPL.
Since then, we have been cleaning up, fixing bugs, and adding features.

Our permanent goal is to create the open source Quake 3 distribution upon
which people base their games and projects. We also seek to have the perfect version of the engine for playing Quake 3: Arena, Team Arena, and all popular mods. This distribution of the engine has
been ported to many new platforms and has had a slew
of new features added, along with massive bug extermination. While we don't have PunkBuster (and never will), we do have
more security for servers and clients from various bugfixes which aren't in id's client.
(Taken from: http://ioquake3.org/?page=home)

Source code is available: http://svn.icculus.org/quake3/trunk/

Portable Quake 3? Yay