Virtual Math Manipulatives--"Thinking Blocks"

http://www.thinkingblocks.com/

I really love the approach on this site: First they teach you how to use the manipulatives to work math problems visually, and then they give you a chance to work it out. For example, the ratios section gives you a word problem and a work space so you can translate the word problem into thinking blocks and figure out the answer.

Great site!