The best free digital manipulatives for math

We know how important hands-on learning can be for students learning to conceptualize mathematics. But in our digital age, students are doing more and more work on laptops, tablets, and PC’s. While no digital tool can replicate the feel of a unifix cube, base-ten block, fraction tile, or protractor, the following digital math manipulatives may work for some of your learners!

Polypad

What others would you add?