From the developers page:

"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."