Digital Calipers
Buying Tool Info
Since you can pick up sets are Harbor Freight for only $15 or so, now's a good time to invest in a pair. The more you work with metal, you'll be glad you have them.
Digital calipers are far better in terms of usability than analog calipers. Digital versions can be switched from metric to standard with the push of a button, often have a memory function allowing you to save the measurement, and provide for zeroing the display at any point along the slide.
When making repetitive measurments, calipers can usually be locked using a lever or thumb-screw.