Modeling Tools

 Blender open source 3D modeling program, complete with physics engine, game engine, and many other features
 SCAD OpenSCAD is a software for creating solid 3D CAD objects. It is free software and available for Linux/UNIX, MS Windows and Mac OS X.
 Google Sketchup fairly easy to use 3D modeler, proprietary
 Cadmium Python library similar to SCAD