Thingiverse offers a variety of ready to print 3-D models. Users can also upload their own designs, customize in existing ones, and participate in challenges.
GrabCAD is an online platform that allows users to view and share CAD files easily. Users also get access to hundreds of thousands of open source models for free.
An extensive database of various resources useful for mechanical engineering as well as designing.
This website provides mechanical and chemical properties of a wide range of materials.