Recently, I became friends with an open source first-person shooter: Quake2World (q2w). As the name indicates, it uses an idTech2-based engine and currently aims to have Quake 2's mechanics.
Although development seems to be centered around graphical features at the moment, the game is sexy and fun. There are no bots so far and only a minimal selection of servers, so you'll have to ask for some company on #quetoo@freenode, which is the community's home.
The project is looking for code and content contributors. The current policy is to allow content licensed under GPL/CC-licenses/Zlib/MIT/WTFPL (although I believe that some of the included content is not compatible with any of these).