here and here. Khan Academy specializes in making sophisticated concepts easy to understand through short video lectures accessed online for free. What started as a simple lectures by Salman Khan on mathematics has expanded to include over 5,000 lectures on a wide variety of topics by a wide variety of faculty. What a bounty of free educational help for students who struggle with concepts, particularly those who don't enjoy good teachers.
The approach has received high accolades, (and of course criticism too about whether the benefits have been adequately proved). I personally have used a few Khan Academy lectures to improve my understanding of certain areas and found them quite helpful.
A recent innovation is the Breakthrough Junior Challenge which invites students between 13 and 18 to submit an instructional video:
The winner this year was Ryan Chester with a thoughtful and coherent explanation of the special theory of relativity. See the video here. You gotta be impressed!