Who would have imagined that a service that makes travelers pay to stay in a stranger’s spare bedroom would be valued at more than Marriott, Starwood, or Wyndham Worldwide? Airbnb did it. The videos that Sal Khan made to teach math to his young cousin were, by his description, “cheaper and crappier” than many other educational videos already online, but they now enable millions of students all over the world to learn at their own pace.