Using multimedia in the classroom is one of the suggestions put forth by math teacher Dan Meyer in his TEDxNYED talk from last spring. I watched the video of his presentation today in preparation for my own talk at this year's TEDxNYED and it's well worth a look by any teacher, not just teachers of math.
If you have other suggestions for interactive web sites, leave them in the comments! Thanks!
1. Interactive Learning Opportunities on the Internet
2. Library of Congress Interactives
3. Spin and Spell
5. PhET Interactive Simulations