These Java applets are a lot of fun. There's also some Windows software to download, and more to explore on the site. If only the whole contents were assembled into an adventure, akin to The Castle of Dr. Brain. I may've showcased these goodies before, but it's more than well-worth experiencing them again. And if you've never played with 'em before, GO!

I enjoyed this--why don't more people realize that math is fun;]