Thinking Blocks is a nice site for elementary and middle school mathematics teachers. Thinking Blocks provides interactive templates in which students use brightly colored blocks to model and solve problems. As students work through the problems they are provided with feedback as to whether or not they are using the correct sequence to solve each problem. There are templates and problems for addition, multiplication, fractions, and ratios. You can also develop your own problems using the modeling tool.
Applications for Education
I originally learned about Thinking Blocks a few years ago when James Hollis posted about it at Teachers Love SMARTBoards. James suggested that Thinking Blocks could be good site for helping students see how algebraic reasoning works. And, of course, if James is writing about it, it must be well suited to use on interactive whiteboards.
Thinking Blocks is also available as a set of four free iPad apps.