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Knowledge is invariably a matter of degree: you cannot put your finger upon even the simplest datum and say this we know.

T.S. Eliot

We are currently in a sort of "online courses" bubble - I am very happy with the quality and always increasing choice - the various universities and schools are competing trying to move more and more stuff online.
Of course some topics are more amenable to go "online" than others, and for the time being you cannot - most of the time - get much in terms of actual "credits".
But this is not important - at least for me - so I am really thrilled by the possibility to pick what I like or what just looks interesting.

Have a look at Class Central - A complete list of free online courses offered by Stanford, Coursera, edX (MITx + Harvardx + BerkeleyX), and Udacity. It is the best way to see what's online and what is becoming available.

Here instead you can find a list of places to check for reviews of online courses (written by people who has actually taken the courses).

The people at CourseBuffet examine and classify courses according to a specific classification system, showing the approximate level a course would be at a traditional university and which courses are roughly equivalent.

Courses I took - take in account that the average retention rate of an Online Course (i.e. how many of the enrolled participants complete the course turning in the minimum required amount of work to actually get a final grade) is around 5%. Typically a course can get 30000 subscribers, and by the end of the course over 28000 will have dropped out.
Many people seem to find the (usually strict) schedule very difficult to keep up with. Personally I have found out that I can manage at best one course at a time but I really shouldn't take two overlapping courses.