Certification

Courses are almost always free. However many providers charge for a certificate of completion. There is also  an issue with MOOCs being accepted for course credit in universities.

MOOC providers with free certificate;

(Coursera, edX, Open2Study, NovoEDStanford Online, Khan Academy)

MOOC with paid certificate;

(Udacity, Coursera, Future Learn, Alison, edX)