These aren't really swappable, though. The Coursera/Udacity/edX courses max out around courses appropriate for freshmen, whereas those linked from math overflow range from freshman year to graduate math courses.

I wonder if Coursera/Udacity/edX will catch up. I can't imagine there is much demand for graduate level math courses on those sites.

https://www.khanacademy.org/ not sure if they are on par with coursera, udacity, and edX though.