Thingiverse
https://www.thingiverse.com/
Thingiverse is a website dedicated to the sharing of user-created digital design files. Download the objects to alter or remix them in the software of your choice, or just browse for inspiration.

Cults
https://cults3d.com/

TinkerCAD and Tutorial

Tinkercad is a free, easy-to-use web app that equips the next generation of designers and engineers with the foundational skills for 3D design.
https://www.tinkercad.com/
https://www.tinkercad.com/learn#/

Blender
https://www.blender.org/
Blender is fully-featured, free, and open source 3D modeling software that works on multiple platforms. It is widely used in the world of professional 3D modeling.
Blender Tutorials
https://www.blender.org/support/tutorials/
A collection of free tutorials to get started with Blender and to explore the more advanced Blender features. Newcomers should start with the “Complete Beginner Tutorial.”

MakerBot
The MakerBot YouTube channel provides videos of real-life applications of 3D printers
https://www.youtube.com/user/makerbot/playlist