An Open Source CSG solid modeling system with ray tracing support for rendering and geometric analysis, network distributed framebuffer support, image-processing and signal-processing tools. http://brlcad.org/
Modelling software that supports NURBS, polygonal modeling, and subdivision surfaces, available for Windows and Macintosh. http://www.solidthinking.com/
A free 3D solid modelling software offering fast hidden line removal, topological functions, 3D geometric constructions, IGES support, shading, printing, hatches, and line constructions. http://www.le-boite.com/minos.htm
Developers of the ActiveSolid 3D solid modeling, CAD prototyping and mark-up sharware application and the underlying BspSolid modelling library. http://activesolid.ncf.ca/