MIT OpenCourseWare is the Massachusetts Institute of Technology’s free online library of course materials. It covers more than 2,500 undergraduate and graduate courses, with lecture notes, exams and video lectures in subjects from maths and engineering to economics and the humanities.

Go to the MIT OpenCourseWare website, search or browse by subject and open any course. No account is needed.

Free with no registration. It is self-study material, so there is no certificate or credit.