Open Thread: What's your killer app?

The other day I was talking with someone about the novel and non-obvious ways that people use Excel in their work and home life. Gotta say, I’ve personally seen some pretty amazing stuff happen when people take a favorite app, get really good at it, then bend it to their will. (And Excel is perfect for this.)

This tracks to Danny’s Life Hack concept by which the alpha geeks were achieving lofty heights of productivity partly by mastering 1-3 “killer apps” – then using them to solve most of their information and functional problems in fairly novel ways.

So my question for you: What’s your killer app? Is there one place where 80% or more of your activity takes place (by choice)? Vim? Excel? Perl? Firefox? Post-it Notes? What’s yours and when did you realize you’d become a badass at using it?