Classzone – my understanding is that this site may be used for educational purposes, even if you are not using their books. Lots of nice animations and learning games.
Project Interactivate – For me, right now, this is the Number One Free Math Site on the Internet for interactive applets! (Otherwise known as the “Shodor Applets.”)
VisualFractions.com – That’s it. Learn about fractions visually. Includes equivalent fractions, mixed fractions, addition, subtraction, multiplication, and division of fractions. Learning games included.
Huge Virtual Manipulatives Site – Click in the body of the table for the standard and grade level band.
Peanut Software – This is great software and it’s free! “Wingeom” does many of the same things as Geometer’s Sketchpad (and some new things too).
Powers of Ten – View the Milky Way at 10 million light years from the Earth. Then move through space towards the Earth in successive orders of magnitude until….(try it!).