Bought a digital caliper to measure stuff - What's your most helpful tool?

I always need bike stuff measured - what is the seatpost diameter, hub spacing, stem size and on and on. I used to ask my mechanic to size stuff, but I just broke down and bought my own digital caliper online. It was surprisingly affordable, and lots of selection from which to choose. This is going to be a very helpful tool for me. What bike tools are a must-have for you?