Ignore all the other answers and skip straight to this one. Math Academy is amazing. It's got everything you could want (mastery based learning, knowledge graph dependencies, assessment tests, a hint of gamification to encourage you to learn more, even spaced repetition reviews) and its coverage and depth of the material is _excellent_.

For specific example, I've literally studied linear algebra probably close to a dozen times in my life, including full courses from two different universities whose names end with "institute of technology", and I'm currently using math academy to review the subject again. I've already encountered several things that are straight up new to me, and the presentation/practice and spaced repetition review of the things I just never managed to lock into my long term memory is already helping me finally own them.