TouchCAD - 3D modeling, unfolding and unwrapping program for Macintosh and Windows. - http://www.touchcad.com/
BRL-CAD - 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/
MINOS - 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
J&L Associates - Developers of the ActiveSolid 3D solid modeling, CAD prototyping and mark-up sharware application and the underlying BspSolid modelling library. - http://activesolid.ncf.ca/
solidThinking - Modelling software that supports NURBS, polygonal modeling, and subdivision surfaces, available for Windows and Macintosh. - http://www.solidthinking.com/