Interactive/Virtual Math Manipulatives - Great Site!
The Utah State University has created a website of interactive/virtual math manipulatives. They broke the manipulatives down by grade level groupings as well as topic area. There are 20 different areas to choose from and many different tools for each area.
Hot Math offers several different features for teachers and students. Most of the site is a pay site, but there are several free portions as well. There is a tuturial for using graphing calculators and some virtual math games.