This comes to us via a tip from Twitter. The Khan Academy has now posted on YouTube over 800 videos (find a complete list here) that will teach students the ins-and-outs of algebra, geometry, trigonometry, calculus, statistics, finance, physics, economics and more. The clips have been recorded by Salman Khan, a Harvard Business School and MIT grad. And to give you a feel for them, we've posted above the first in a long sequence of lectures on differential equations. (The remaining lectures can be found here.) This YouTube channel, which now appears on our list, Intelligent YouTube Video Collections, is one of several video sites that provide free online tutoring via video. As mentioned in the past, you can find online good video collections dedicated to chemistry and calculus.