OpenArena is a game based off of the Quake III Arena source code. It was intended to be a completly free version of Q3A but with assets provided for free under the GPL v2 license. Gameplay is almost exactly the same as in Q3A, but all the art is different and hitscan weapons are unlagged. Some people don't like the lack of a consistent theme, but I don't find that to be much of a problem, especially on the non-OA maps that the servers provide. Not all textures from Q3A have been replaced, so some maps have missing textures. This can be fixed with texture packs that some servers provide. The main problem I have with the game is that some of the models are not what I would call modest. In this respect, OA is worse than Q3A. Supposedly the next release of OA will fix this, but who knows when that will come out. If you really can't tolerate them, it should be possible to replace the skimpy models with other ones such as Sarge or Major or force player models client-side.
If you are a modder, OA is not that much different than Q3A. The main problem that I have encountered is that nearly all the example code online is licensed under the old Q3A license, which is not compatible with the GPL. Of course, this is a problem for the GPLed Q3A source code as well.
GPL Mods and Resources
My unofficial OpenArena engine that periodically is updated to the most recent release of ioquake3.OpenArena ioq3
Source code for the excellent Ratmod. Not my own. Features projectile delag.
Ratmod
Not a mod, but a set of scripts for Defrag that records top times and speeds for each map.
OpenArena Defrag Stats
Main focus is physics, but I may add other stuff as well.
Origami Mod
GPL v2+ CPM physics tutorial
GPL v2+ CPM physics mod
GPL v2+ Defrag-like mod
Explanation of the overbounce bug(s)
List of physics bugs