DragonBox is one of my favorites to teach basic algebra to my eight year old nephew: http://www.dragonboxapp.com/. It came out of a research project at U of Washington.

I also love doing Maker projects because I believe the best way to teach math is to build real-world, functional things that show the end purpose of math. For example, I have done this workshop with my nephew too which really engaged his interest in math and STEM more broadly: http://www.einsteinsworkshop.com/now-enrolling.