Shapeshop Free Modeling

Shapeshop is a Windows-only modeling application, for 3D sculpting and sketching. It's not CAD and it's not ZBrush, but it tries to be easy to use.

Check http://www.shapeshop3d.com

You can export models in OBJ, Collada and STL format for use in other applications.

Modeling only, so full texturing, rendering and animation is done in other software.