I love how websites like Coursera pioneered the abundance of free MOOCs by many awesome institutions and professors.
I have not taken many MOOCs so far but one I did take and quite liked was Learning How to Learn. Although I liked Mind for Numbers book more. The course is also based on the book.
Has some great courses
hn.academy - Collects, ranks, and showcases the online courses recommended by Hacker News users.
Class Central - Search online courses.
Search engine for lectures (HN)
The best online courses I invested in this year (2019)
Ask HN: What makes an online course amazing? (2020)
600 free courses from universities (2018)