Thingiverse

The most popular community site for 3D models! People are encouraged to license their creations under CC, so many of the items here can be remixed.

http://www.thingiverse.com

“MakerBot’s Thingiverse is a thriving design community for discovering, making, and sharing 3D printable things. As the world’s largest 3D printing community, we believe that everyone should be encouraged to create and remix 3D things, no matter their technical expertise or previous experience. In the spirit of maintaining an open platform, all designs are encouraged to be licensed under a Creative Commons license, meaning that anyone can use or alter any design.”