Red Eclipse is a first person arena shooter, suitable for casual gaming. It's built on Cube Engine 2 and runs on all major platforms. It has a few built-in game modes, adaptable settings, comes with a level editor, and lots of community-maintained modifications or patchsets.
bEngine is a game engine for browser-based and 3D/OpenGL graphics rendering. It provides for space ships, buildings and terrains, transportation and resource aquisition features, but can be extended with additional modules.
Unvanquished is a first person shooter with elements for realtime-strategy games. It's derived from Tremulous, and adapted to the Deamon-Engine. It pitches adaptable aliens against technologically advanced humans, with either team being playable. Humans get credit for new weaponry, whereas aliens get an evolution score to progress.