Collection of Science Demonstrations
This is an amazing resource: a collection of science demonstrations from Harvard Natural Sciences Lecture Demonstration Services.
There are hundreds of demonstrations in the archive. Each entry includes an in-depth description of the scientific principle(s) on display in the demonstration, as well as detailed instructions on how to execute the demo.
Topics include Newtonian Mechanics, Light and Optics, and Thermal Physics. There is also a collection of mathematical demonstrations, as well as a small set of videos.
And be sure to check out their Facebook page here: http://www.facebook.com/NatSciDemos