is a collection of mechanical models and related resources for teaching the principles of kinematics–the geometry of pure motion. The core of KMODDL is the Reuleaux Collection of Mechanisms and Machines, an important collection of 19th-century machine elements held by Cornell’s Sibley School of Mechanical and Aerospace Engineering.
The site is very cool, with many photos of models, videos, tutorials and general information. On the references session you can read important documents and papers, like Codex Madrid I by Leonardo da Vinci, on PDF or online. Very interesting. (via MetaFilter)