One tool every gearhead should have and why

Hear is a good one, its not exactly in the tool box but on my list, I guy comes in to my store the other day and he has ( and is selling for 300 bucks CDN ) a small thermo imaging camera that plugs into your cell phone it is about 1" square he uses it for looking for hot spots in wires, coils, cyls etc, the image is projected directly on your smart phone screen, now I gotta say I am not a geeky tech guy at all but that is cool as hell and on my list particularly useful with multi coil set ups on some of the engines I work on.