It is old news by now, but for those of you who did not know (and especially for all students among you, undergraduates and graduates alike), MIT made available online extensive material for no less than 1800 courses from their curriculum. Inter alia, you can’t miss:
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The MIT economics courses available online
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MIT’s music and theater arts courses (e.g., here’s a very interesting course on hip hop and here’s what I found most interesting)