For me, Keyboard Maestro[1], which is probably the one app I couldn't live without.
With it, I can do things like highlight any git sha (or any sha range) in the terminal, press a keyboard command and it will launch my diff tool with the diffs for just that commit.
I also use it to do inline transpiling of different code. I have keyboard commands for transpiling my highlighted Sass or ES6 code and replacing it with the result.
It's so powerful, there's very little it can't do.