I often recommend this app in person – DragonBox Algebra 12+ - The award-winning math learning game Review

Whenever someone mentions they have kids in school, I ask if they've heard of the DragonBox apps. They teach the spirit and the practice of math without assuming you are going to memorize a long list of weird rules just because an adult told you to.
Review by l33t pope street on DragonBox Algebra 12+ - The award-winning math learning game.