Netfabb - Innovative software solutions for 3D printing and additive manufacturing.
LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
FreeCAD - An open-source parametric 3D modeler
meshmixer - meshmixer is an experimental 3D modeling tool for making 3D mashups without too much hassle.
SketchUp - 3D for Everyone