Udacity – I have taken a few Udacity courses over the years, and I have found that them to be quite successful. In addition to “one-off” courses, they also offer what they call nanodegrees, which provide a useful credential for the student.
Georgia Tech OMCMS – Georgia Tech’s Master’s programs in computer science are built on the Udacity platform and many (if not all) are freely available as a MOOC, though one obviously needs to be enrolled in the program for graduate credit.
Khan Academy – One of the originals and still hard to beat for math education. I used some of the videos as course content and extended resources in my own online math course.
Lynda (now LinkedIn Learning) – Another of the early online learning resources, and still a great place to go to get accessible learning on a wide variety of subjects.
Lamar DLL – At the risk of sounding like a brown-noser, I have learned much about effective course design as a participant in the DLL courses.