Explain 3D - 3D Animations of Simple Machines

Explain 3D is a free site that offers a small of collection of 3D animations of simple machines. While the animations are free to view, you do have to register in order to see them. The animations use the Unity web player which enables viewers to zoom-in, zoom-out, and rotate animations 360 degrees.

As you can see in the screenshot above, there are eight Explain 3D animations. I'm not too keen on the machine gun animation, but the rest could be useful explanatory aids.