MIT OPENCOURSEWARE: https://ocw.mit.edu/index.htm
Khan Academy: https://www.khanacademy.org
Excellent online educational sites to provide a free world class education for all. I like their content.
Besides, for those who are mastering English as their foreign language, perhaps the best way to do so – especially at the intermediate and higher levels – is studying some subject in the English language, NOT just “general English.” In this sense, such resources are also invaluable.