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.
Bradley renderings through rtwizard
Hubble Space Telescope rendering
Mike Muuss working with BRL-CAD on the PDP-11/70
Screenshot of MGED and moss.g
Small bicycle wheel. Model downloadable from "Geometry"
Stryker ICV w/ Slat Armor rendered with ADRT/RISE