Trevor Blake sent me this:
LessWrong is “community blog devoted to refining the art of human rationality.” I’ve occasionally dipped into the blog, but never made much of a habit of it. But this reference page is excellent – the section on mathematics seems particularly useful. There are sections on artificial intelligence, machine learning, game theory, computer science, philosophy and more.
And via that resource page are two other amazing resources:
Khan Academy: A massive collection of free self-paced math and science lessons.