Pardon the geek-out, but I'm just a little bit too excited about the latest addition to my tool kit:
That, for those not in the know, is a vernier caliper, which (if you aren't in the mood to read the whole wiki) is used to measure stuff. More precisely, it's used to measure stuff more precisely. I don't know how I've called myself a mechanic for this long without owning one. Not sure if the clamp diameter of your handlebars is 25.4mm or 26.0mm? Bust out the vernier caliper. Trolling seatposts at a swap meet and find one without a diameter marking? It's vernier time, baby. Hang around with the sort of people who simply MUST know the actual, inflated width of Brand X, Model Y tire? The guy with the vernier caliper is the life of that party -- though, as you might imagine, it's a pretty lame party.
At my first serious shop gig, our head mechanic had his own personal (and very pricy) vernier caliper, purchased with his own money, that no one else was allowed to touch. I think he even took it home every night (there was speculation amongst the more junior staff that he couldn't sleep if it wasn't on his bedside table). But if you needed something measured -- and I mean you really needed it measured right, you took it to him. He'd pull that gleaming, polished steel from its leather case (seriously, it came with a leather case), close it gently around the part in question, squint at the tiny increments like a Talmudic scholar poring over a particularly thorny text, and proclaim The Measurement to more decimal places than you probably needed or even imagined possible. It was glorious.
Mine's an el cheapo, thirteen U.S. dollars worth of pretty rough looking metal (couldn't bring myself to pop for one of those high-precision Excalibur numbers with the leather case), but I'm still in full nerd mode. In fact, I'd write more, but there's a whole house full of stuff just waiting to be measured.