A free, portable 3D engine with a ton of features.

Crystal Space is currently under heavy development, but it's already quite impressive. The engine supports stencil shadows, lightmaps, shaders, portals, mirrors, scripting, 3D acceleration and lots more. It compiles and runs on GNU/Linux, Windows and MacOS/X.

The Crystal Space project's aim isn't to produce a game, but rather a flexible, portable 3D engine.

License: free