Khan Academy was a site mentioned during one of the sessions (can't recall exactly which one) of Connecting Online 2010 (CO10). The following is a brief introduction from the actual site:
"The Khan Academy is a not-for-profit organization with the mission of providing a high quality education to anyone, anywhere.
We have 1000+ videos on YouTube covering everything from basic arithmetic and algebra to differential equations, physics, chemistry, biology and finance which have been recorded by Salman Khan."
Here is a promotional video: