Khan Academy is one of the best collections of educational videos on the web.
Khan Academy makes it possible to build a "perfect crystal" of knowledge in an incremental self-paced fashion.
Khan Academy is an example of a tool that makes it possible to implement a great idea: "flipping the classroom".
If you see research questioning the value of Khan Academy, start from answering the basic question of such an assessment: is research based on paid volunteers or students who choose their videos on their own for their own pleasure of learning?
Go to: https://www.khanacademy.org