I've signed up for lots of MOOCs just to check out what is being covered, or what has changed since I took a class in college. I've also finished a couple of MOOCS that I thought were useful. I've also registered for MOOCs just so I can download the course materials to go over someday when I have spare time. I wouldn't count the times I didn't participate and finish the course as failures, I was just using the class in a different way.

A whole beowulf cluster of them.

I like to see comments describing the data structures. If I know what variables mean it's usually not too hard to figure out what a piece of code does. It's much harder to do it the other way around.