Blender - Blender is the open source, cross platform suite of tools for 3D creation.
FreeCAD - An open-source parametric 3D modeler
SelfCAD - SelfCAD is an online 3D modeling and slicing software.
meshmixer - meshmixer is an experimental 3D modeling tool for making 3D mashups without too much hassle.
SketchUp - 3D for Everyone
Onshape - Onshape is the first full-cloud 3D CAD system. It runs in a web browser and on any mobile device.