Udacity – http://www.udacity.com/
“Udacity was founded by three roboticists who believed much of the educational value of their university classes could be offered online. A few weeks later, over 160,000 students in more than 190 countries enrolled in our first class, ‘Introduction to Artificial Intelligence.’ ..”
MIT Open Courseware – http://ocw.mit.edu/index.htm
“Unlocking Knowledge, Empowering Minds. Free lecture notes, exams, and videos from MIT. No registration required.”
Learning Space via Open University:http://openlearn.open.ac.uk/
“Over 600 free online courses from The Open University. Available from introductory to advanced level, each takes between 1 and 50 hours to study.”
Note: the sites are arranged in no particular order.