Looking to boost your AP or SAT score, or to understand some aspect of physics or calculus that your teacher never properly explained? Looking to learn something new such as world history, calculus or economics.
Check out the Khan Academy.
Its mission is to provide world-class education, for free, to anyone, anywhere. The Academy was founded by MIT and Harvard graduate Salman Khan who has recorded thousands of videos on math, science, finance, and economics at khanacademy.org. Khan Academy has partnered with the College Board for free SAT prep.
The Academy currently offers thousands of videos on YouTube, with the goal of eventually providing a full-featured, free virtual school covering all major subjects. The current library offers a range of subjects from basic arithmetic to calculus, chemistry, physics, finance and SAT prep. There is also a web-app that generates practice problems.