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/

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/

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

RogCAD
   Keyboard driven modelling software that supports complex curved surfaces.
   http://rogcad.com/

TouchCAD
   3D modeling, unfolding and unwrapping program for Macintosh and Windows.
   http://www.touchcad.com/

solidThinking
   Modelling software that supports NURBS, polygonal modeling, and subdivision surfaces, available for Windows and Macintosh.
   http://www.solidthinking.com/