Assetforge - Asset Forge allows anyone to create 3D models and 2D sprites using building blocks.
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
Lightwave 3D - LightWave 3D combines a state-of-the-art renderer with powerful, intuitive modeling, and animation...
SketchUp - 3D for Everyone