Thingiverse - A design community for discovering, making, and sharing 3D printable things. All designs are encouraged to be licensed under a Creative Commons license, so anyone can use or alter the design.
National Institute of Health 3D Model Exchange - Provides models in formats that are readily compatible with 3D printers, and offers a unique set of tools to create and share 3D-printable models related to biomedical science. Access free designs by selecting "Public Domain" under License dropdown