Before learning it though, I'd like to get inspired about what's possible. Thus collecting a list of cool stuff built with Javascript. What cool apps/websites/experiments that you know of have been built with Javascript? List them here.
Fallout Hacker Helper http://faehnri.ch/fhh/ - a utility to help with the hacking mini game in Fallout 3
WRAP http://faehnri.ch/wrap/ - Web Register Assembly Programming, a browser version of Daniel Dennett's RAP machine, a simple register machine and language he uses for some theory of mind concepts (read his Intuition Pumps)
Disrupt Inspiration http://faehnri.ch/disrupt-inspiration/ - something I made quick after a friend and I were joking about startups. Generate startup ideas in the form of "X, but for Y." Submit ideas!
My point is: almost anything can be build with Javascript.