These websites are the major repositories for finding and downloading free 3D models for use with a 3D printer. The sites files range from very simple free 3D models to very complex 3D models.
3D ContentCentral hosts free 2D and 3D CAD files from engineers and firms. You can also request models.
The Shapeways 3D parts database is a place where you can download free 3D models free for 3D printing. The files currently in the database have all been tested and printed by Shapeways and all “print for sure.”
Operated by Defense Distributed, the site was a makeshift response to Makerbot Industries’ decision to censor firearm-related files uploaded at Thingiverse. They are started as a host of firearms related files, but have now expanded to the inclusion of ANY file, regardless of intellectual property rights.
GrabCad is a site for engineers to show off and share their CAD works, with over 75,000 free models available. .stl filter available.
TurboSquid is a large repository of both pay-for and free 3D models. There’s no area dedicated specifically to 3D printing, but there are selectable filters such as .stl and .sldprt (Solidworks part).
Thingiverse is a huge community of people who create and share 3D designs freely, so that all can benefit from them. Widely used by the RepRap Project, and by 3D Printer and MakerBot operators.
TraceParts is a huge 3D parts database founded in 1989 with some very complex CAD files available in numerous formats, including .stl.
The Trimble 3D Warehouse (formerly Google 3D Warehouse until purchased by Trimble) is a free, online repository where you can find, share, store, and collaborate on free 3D models created in SketchUp. This is a great place to look for models of famous buildings, as SketchUp interacts with Google Earth.