OpenSCAD
The Programmers Solid 3D CAD Modeller

OpenSCAD is a software for creating solid 3D CAD models. Unlike most
free software for creating 3D models (such as 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.