The tool you reach for more than you expected

Some tools earn their place on the bench quickly. Which modest tool turned out to be more useful than you expected?

A small inspection mirror. It has saved me from taking things apart just to check the far side of a bracket.

A notebook that stays in the shop. Measurements and sketches are much easier to find when they all live in one place.

A decent task light. Being able to see the work clearly makes almost every job less frustrating.