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 was watching 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 math teachers.
If you have other suggestions for interactive web sites, leave them in the comments of my blog located at http://www.lucygray.org
1. Interactive Learning Opportunities on the Internet
2. Library of Congress Interactives
3. Spin and Spell
5. PhET Interactive Simulations