Unlike regular college/ university courses, MOOCs can attract many thousands of enrollees around the world. They can come in the form of active course sessions with participant interaction, or as archived content for self-paced study. MOOCs can be free, or there can be a charge – either on a subscription basis or a one-time charge. Free MOOCs sometimes have a paid “verified certificate” option.
— The 50 Most Popular MOOCs of All Time http://www.onlinecoursereport.com/the-50-most-popular-moocs-of-all-time/
A good list, but be sure to read the caveats at the beginning and end.