OpenSCAD is software for creating solid 3D CAD models. Specifically designed with 3D printing in mind, OpenSCAD is one of the most widely used tools for designs published on Makerbot Thingiverse (more than 5000 designs), and the most widely used RepRap designs (Prusa Mendel i2 and i3) are created using OpenSCAD. 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. OpenSCAD is multi-platform and Free Software, licensed under GNU GPL v2.