Blender - Blender is the open source, cross platform suite of tools for 3D creation.
LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.
Sculptris - Sculptris: Enter a world of digital art without barriers.
FreeCAD - An open-source parametric 3D modeler
Misfit Model 3D - Misfit Model 3D is a 3D model editor that basically operates with triangle-based models.
SketchUp - 3D for Everyone