Blender - Blender is the open source, cross platform suite of tools for 3D creation.
FreeCAD - An open-source parametric 3D modeler
Sculptris - Sculptris: Enter a world of digital art without barriers.
LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.
ShapeShop - ShapeShop is an advanced and professional 3D design and modeling application that comes with an easy-to-use and user-friendly interface.
SketchUp - 3D for Everyone