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.

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