Blender - Blender is the open source, cross platform suite of tools for 3D creation.
Clara.io - Free online 3D modeling software: Design, editor, animation, rendering, & lots of free 3D models. Supports Three.JS, Blend, FBX, OBJ, V-Ray, Babylon.JS, STL.
FreeCAD - An open-source parametric 3D modeler
SelfCAD - SelfCAD is an online 3D modeling and slicing software.
Sculptris - Sculptris: Enter a world of digital art without barriers.
meshmixer - meshmixer is an experimental 3D modeling tool for making 3D mashups without too much hassle.