Reaction (formerly called Reaction Quake III) started out life as a total conversion mod for Quake III Arena but has since morphed into a standalone game built atop the open-source Quake 3 engine code-base in the form of ioquake3. Reaction is available freely and the content is licensed under the Creative Commons. After being in development for a long time, Reaction 1.0 Beta is now available for Linux, Windows, and Mac OS X platforms.
This announcement comes at the same time Quake2World was released. It's too bad though there aren't any new open-source games to report on yet using the iodoom3 engine.
The Reaction beta announcement along with other information is available from RQ3.com.