One tool every gearhead should have and why

A test light, well just because. No really its a quick way to test circuits and fuses etc.. It will verify power and ground quickly.

Also a hammer, well just because it feel good bashing something sometimes...they guy in the bay next to me used to get nervous when I used to say "I'm fixing to take this ball peen hammer all down the side of this piece of chit" when referring to a troublesome customers car...