Watch, Practice, Learn Almost Anything for Free
With the stated mission “of providing a world-class education to anyone, anywhere”, Khan Academy makes available over 2600 videos on the topics of math, science, history, and finance. Each instructional video is anwhere from 10-20 minutes in length. Math is currently covered the most extensively, topics suitable for K-12, but creator Salman Khan says his goal is to eventually cover everything.
Khan Academy web site
PBS NewsHour piece on Khan Academy
Khan Academy, with Khan as the only teacher, appears on YouTube and elsewhere and is by any measure the most popular educational site on the web.