Also Saylor org
There are quite a few more, too, actually. I recently finished one from Saylor. I have a Ph.D. and don't want any more degrees but I love learning, and at least with the course I took, when you finish, you take an online final and if you pass, you get a certificate. Many of these courses (including Saylor) provide ways you can actually get college credit for them, but I have not checked fully how that works since I don't want or need the college credit.
They may give an additional test and I'm sure there are some charges involved, but overall it is a very inexpensive way to learn if you are self-motivated.