In actual manner what I say about khan academy and their work because I think what ever I would say about khan academy would be less.
Their techniques to teach in any stream ( either it's a Organic Chemistry of 12th standard or Physics or Math or Economics or Entrance level Exam preparation) are very excellent and very pleasant.
Another good point which I have personally noticed that in their each and every video lecture they cover the topic related concept with - in 15 to 16 minutes which is a very-very good concept. I personally thanks to all of the members of khan academy.