Voice of the Masses: What’s your favourite CLI tool/trick?

A new Linux Voice podcast looms upon the world like a giant, looming thing. For this episode, we want you to tell us: what’s your favourite trick, tip or shortcut at the command line? Maybe it’s a mega time-saver key-combo that has changed your life, or some nifty little app you found on GitHub. Whatever the case, post in the comments below and we’ll read out the best. Yes!