Your post got me thinking: what other things can one learn that give ridiculously good return on time invested? Not just technologies, but anything. Here's an incomplete list:

* Graph flow: a lot of interesting problems can be solved efficiently by converting them to graph flow problems.

* Basic microeconomics: the world looks different once you understand it, and it's not that hard to learn. A cheap enlightenment boost!

* Monoid-cached trees. Guy Steele explains better than I could. Video and slides:

http://vimeo.com/6624203

http://labs.oracle.com/projects/plrg/Publications/ICFPAugust...

Any others? I'm sure I haven't done more than scratch the surface.