Wolfram Demonstrations (Math/Illusions/Science)
I was looking around at some of my old bookmarks, and found an interesting site Wolfram, the makers of Mathematica. They have an area they call Wolfram Demonstrations Projects which are interactive mini applications made in Mathematica.
The idea is that developers can program a "notebook" which allows users to control different parameters - change the parameters and you get different results. The site includes animations, but if you get the free Mathematica Player, you can download files, then manipulate them all you want.
Demonstrations include mathematical concepts like manipulating polygons, but also more real-world concepts like optical illusions, how fires spread, graphic tests and more. Each page includes a brief explanation of what's happening and links to further information.
Some of my favorites are
Best of all, if you download the files, you get a link to the Mathematica source code. Penn State has a site license for Mathematica, so I'm investigating to see if I can do any interesting modifications.
P.S. My only complaint about the site is that it's hard to find some topics. The Site Index lists everything in alphabetical order.