What’s your favorite tool few know about?

I'll go first - TextExpander. it's a tool I use everyday that lets you write keystroke shortcuts for templates, urls, saves notes and really anything else. Essentially it's like an outreach template that you can use in any application. It saves me tons of time and also means I'm prepared for any question or onbjectiok because I save the snippet once and then just activate it when I need it.