Ask HN: What is a good website/app for math practice?

Hello friends,

    I am looking for a website or app good for practicing math with. Ideally it would have an interface like taking a test, where it gives you one multiple choice problem and after solving gives you a new one. I would like to be able to have practice questions for anything from high school Algebra 1 all the way to advanced mathematics like calculus and linear algebra. Any thoughts?