Blender is a powerful full featured open source 3d app. It runs on windows, linux and apple.

The main Blender page can be found at: http://www.blender.org/

Download: http://www.blender.org/download/get-blender/

An excelent resource for getting started with Blender:

http://en.wikibooks.org/wiki/Blender_3D:_Noob_to_Pro