Sauerbraten (a.k.a. Cube 2) is a free multiplayer/singleplayer first person shooter, built as a major redesign of the Cube FPS.
The engine supporting the game is entirely original in code & design, and its code is Open Source (ZLIB license, read the docs for more on how you can use the engine).
In addition to the FPS game which is in a very playable state, the engine is being used for an RPG which is in the preproduction phase. Additionally, Proper Games ltd are the first to use the engine commercially.
Official website: http://sauerbraten.org/