OpenSCAD is a software for creating solid 3D CAD objects. Unlike
most free software for creating 3D models (such as the famous
application Blender) it does not focus on the artistic aspects of 3D
modelling but instead on the CAD aspects. Thus it might be the
application you are looking for when you are planning to create 3D
models of machine parts but pretty sure is not what you are looking
for when you are more interested in creating computer-animated movies.
OpenSCAD is free software and available for Linux/UNIX, MS Windows and
Mac OS X.