Coursera is phenomenal. I've taken history courses, nutrition courses, international development courses, teaching professional development courses ... the quality will vary since the courses are coming from a myriad of different institutions, but they're all quite good. You can involve yourself heavily in the course - doing all the assignments and participating in conversation boards - or you can simply audit the courses. You can also download the video lectures so that you can revisit the things you learned at a later date. I don't think they have any language courses, but they do have courses offered in other languages if you're simply trying to maintain or improve a language you've already learned. And it's free, so there's no reason not to try enrolling in something that catches your eye.
