The game is self described as:
Red Eclipse is a Free and Open Source game, using SDL and OpenGL which allows it to be ported to many platforms; grab a development copy from our Subversion repository and live on the bleeding edge.
The game is a single-player and multi-player first-person ego-shooter, built as a total conversion of Cube Engine 2, which lends itself toward a balanced gameplay, completely at the control of map makers, while maintaining a general theme of agility in a variety of environments.
It's available for Windows, Mac OS X, and Linux/BSD. It can be downloaded at RedEclipse.net.