In my case, I would probably say command line programming in awk. It's something you can master in an afternoon or two and that will save you countless hours for decades to come.
Interpersonal skills (re: AwesomeFaic), grokking bureaucracy, learning how to learn are all career-long pursuits that define most careers (I would argue).
Tools like awk and sed are great but ultimately of marginal importance to the average developer. In fact, I haven't even found the sum of all similar efficiencies to be worth dealing with.