solidThinking

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

Touch-3D

3D modeling, unfolding and unwrapping program for Macintosh and Windows, and free-form add-on MCnurbs Creator for VectorWorks and MiniCad.
http://www.algonet.se/~ludesign/

RogCAD

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

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

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/