OpenSCAD
Description
OpenSCAD is a free, open source program that is used as mathematical, script-based 3d design software. Unlike the others on this list which are very "mouse-based", OpenSCAD uses simple code commands to create 3d models using basic mathematical relations.
Availability
OpenSCAD is open source software, anyone can download and install it for free on their personal computer.
http://www.openscad.org/downloads.html
Getting Started
Tutorials are available at http://www.openscad.org/documentation.html