- Thingiverse
- 3D Content Central. http://www.3dcontentcentral.com
- GrabCAD. https://grabcad.com/library
- 3D Part Library. http://www.3dpartlib.com/
- 3D Warehouse. https://3dwarehouse.sketchup.com
The ones for CAD are more useful, because they usually have the dimensions right. You download those because you want the parts to fit. For competitive reasons, they tend to be good looking models, even though that's not strictly necessary.