Khan Academy
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Worst math website ever, only upside is that it's free.
If I fail a singular question, they make it impossible to get a complete mastery on the course. On the Course Challenge, a test with 30 questions, if you get a single answer wrong, everything is useless and you have to do it all over again, and there isn't even a feature that allows you to restart it. You have to manually go through every other question, which will give you a horrible record on the Course, only to restart it for another shot at acing the test, and if you still get one question wrong, you have to do it again.