Khan Academy

Sal Khan started recording math tutorials for his cousin in 2008. Now Khan Academy serves 150 million registered users across 190 countries — completely free. Their mastery-based learning system adapts to each student, and their SAT prep course (built with College Board) is the best free test prep available. Khan Academy proves that world-class education doesn't need a paywall. The nonprofit model works because it's backed by Gates Foundation, Google, and millions of small donors.