I want to learn something new

I work at a computer all day long and have lots of short snippets of free time between tasks. There is no pressure here to look busy. I usually just goof off surfing the web and when I think of the years I wasted which I could have spent mastering something new I feel ill.

So here’s the deal:
-something accessible on the computer is preferred, but it doesn’t have to be, desk space here is limited though.
-I have headphones
-I have to be able to put it down often and pick it back up, I have anywhere between 5 to 25 minutes of available time between work stuff.
-Free is better, but doesn’t have to be.
-I’d like it to be useful

So far I have been doing a steady regiment of Spanish lessons at duoLingo, a Typing Attack game to brush up my touch typing skills and I’m getting started with YouTube tutorials to learn GIMP.

Any ideas?

There are several colleges that post courses online.

Harvard University
MIT OpenCourseWare
University of Oxford
stanfordonline
UCBerkeley
UCLA Courses
YaleCourses