BRL-CAD is a powerful cross-platform constructive solid geometry solid modeling system that includes an interactive geometry editor, ray-tracing for rendering & geometric analyses, network distributed framebuffer support, image & signal-processing tools.
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.