I'm not using anything regularly though I'm sure they're helpful. I had a few languages in college but not enough to become fluent. So am watching a lot of French TV and movies and trying to get my hearing comprehension up to speed. Or watch a movie in English with French subtitles...which is really helpful.

I like math but had way too few classes so have been working my way through calculus via a free online course.

Both Harvard and MIT have some really interesting free-or-cheap courses including one on the science of cooking. I find it fascinating as I don't have the patience to read the books on the subject.

Are any others taking online courses they want to recommend?