Rightstart Mathematics. I went through calculus in high school and teaching my elementary aged children with this program has taught me more math and helped math make sense in ways I never thought possible. That said, it is very parent-intensive time wise. In my opinion, the benefits are worth it!
Organizing RightStart Math cards and card games
9 math YouTube channels for school age kids to learn math, and to be fascinated by math thinking and ideas. Great math resource for math class, math tutor, math homework help, or homeschool math teaching.