One tool every gearhead should have and why

This might be a fun and informative thread. Please name ONE tool you think every gearhead should own, and why. I'll start with a digital thermometer. I paid about $90 for mine when they were fairly new, but they are down to about $20 now. I have had mine for several years. It comes in very handy when you want to tell if a thermostat is working. Aim the thermometer at the thermostat housing. When the thermostat opens, the temp will jump. For example, if you have a 180 degree thermostat, the temp may jump from 130 to 180. That way you know your thermostat is working as it should you can also easily tell if a cylinder isn't firing right. If the header tubes near the flange on 7 cylinders is 350 - 400 degrees and the 8th is only 200, there is a cylinder that isn't firing right. Plus, it's fun to use on a summer day to see just how hot the pavement really is.