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.
Sculptris - Sculptris: Enter a world of digital art without barriers.
MatterControl - MatterControl is a free, open source app that lets you organize and manage your 3D prints.
SketchUp - 3D for Everyone