TurboSquid - TurboSquid offers 3D models for professionals.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
STLFinder - The 3d models search engine.
FreeCAD - An open-source parametric 3D modeler
3DOcean - 3D Models & Accessories From $1 for 3D Models, Textures, Materials & Shaders, 2D Concepts...
MATLAB - A high-level language and interactive environment for numerical computation, visualization, and programming