The following group are not for profit and allow free enrollment. (You may need to make a payment for a university or high school equivalency certificate.) Generally each course starts on a particular date and you are encouraged to keep up with the class by viewing the lectures and doing the course work each week.
Future Learn - Mostly British-based universities.
EDX - another group of well-known universities, I have signed up for the "Science of Everyday Thinking" course, a fairly lightweight psychology course. Previously I have completed "Introduction to Biology (The Meaning of Life!)" and "Our Energetic Earth". The latter, from University of Toronto, was interesting but lightweight, with a talking head lecturer plus slides and some recommended texts. The former was the wonderful Generitcs, Biochemistry and Genomics course I mentioned. Live lectures and a gifted lecturer. Fantastic software tools in the homework (as you'd expect from MIT, I suppose).
Khan Academy has mostly pre-university courses, but they seem to be conducted in a very approachable way. Emphasis on mathematics. Self paced. I have viewed a biology and world history course accessed through this portal and it's a lot of fun! Also on YouTube: Crash Course Biology. Videos with a quirky approach that would appeal to teenagers, I think.
Academic Earth have pre-recorded mostly university lectures. I have just started one called Oceanography, which is recorded from the back of a darkened lecture hall with a computer-generated slides projected on a screen. It's just like being there, sometimes out of focus and all :-) I'm glad I didn't pay for that university course, but perhaps it will pick up.
Iversity is a German-based MOOC but they have a number of what appear to be good courses in English. I vill let you know vhat I zink of zem :-)
Saylor.org has a set of on-line courses that are contributed by experts and seem to consist mostly of self-paced directed readings of on-line materials with occasional viewing of videos. This is more of a traditional approach to a "correspondence course". It probably works well for people who like reading.
The following is a for-profit company but they allow you to do the courses for free and give you a certificate with an extra payment for "validation" i.e. university-equivalence. So basically the same results as far as the user is concerned.