Although not a "tool" my favorite is the battery disconnect knob. It makes working on the car so much easier turning off the juice by twisting a knob. I have the one with the bypass to keep the clock and radio presets intact.
A 1/2" drive comes in handy on the suspension bolts, as do the handle extensions for more leverage. Those Stuttgart "gorillas" really know how to tighten down those bolts.
The Black & Decker "Snake Light" is handy when working under the dash or under the car.