Stanford Online. Interestingly, these courses run on the edX platform (but are offered directly by Stanford, not edX). Hopefully that means the quality control will be better.
I probably won't complete both of them (I've got my doubts about How to Learn Math but I'm always up for new perspectives on teaching), but it will be good to do some mental stretching after a cramped academic year.
The observant among you will notice that neither of these courses have anything to do with MOOCs or technology. That's not an accident.