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.
A very, very klunky interface. E.g. to run brl-cad on Ubuntu, you need to run mged from the command line. The process of changing attributes is also very tedious. This would have been a great package in the 1980's.