Armed for writing

A baker's favorite tool may be their KitchenAid. A construction worker's favorite tool may be their hammer. A parent's favorite tool may be a portable gate. What about writers? My favorite tool to be armed with is a thesaurus. There's only so many times someone can smirk before you want slap it off their face. Or maybe you want to describe how happy someone is but excited doesn't cut it. You look it up in the thesaurus and decide ecstatic is a much better fit.  What's your favorite tool that you make sure is nearby your writing station at all time?