cmftStudio - GUI counterpart of cmft - cross-platform open-source cubemap filtering tool.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
open3mod - A free, Open-Source 3D Viewer designed to make it all easier. Massive 3D Format support
FreeCAD - An open-source parametric 3D modeler
Online 3D Model Converter - free online 3D model converter, upload a model and export it into popular formats
meshmixer - meshmixer is an experimental 3D modeling tool for making 3D mashups without too much hassle.