Crystal Space is an Open Source 3D SDK for Unix, Windows, and MacOS/X. It renders with OpenGL and features GLSL shaders, CG shaders, deferred rendering, dynamic shadows, bullet based physics library, terrain engine, skeleton based animation meshes, exporter for Blender, portals, etc...
The Open Descent Foundation (ODF) is a community for developers, who want to help to create open source programs, tools, etc. for the famous games Descent1, -2 or -3. Everyone can join the ODF if he accepts the idea of writing open source programs.
MusicWatcher is a perl based non-realtime music analysis and visualization system with complete example implementations. One example renders a 3d music driven animation in povray.
Lamia or Lamia Engine is the modern generation free distributable and open source game engine for creating computer games for several prevailing software platforms and very maybe hardware platforms either.
spyGLass is an OpenGL call tracer and debugger. It goes a step further than simply using ltrace because it knows what the arguments to the various gl* calls are, and it also knows about OpenGL's error flag.