Measuring Tools
The core set of tools for taking body measurements and checking pattern accuracy — tape measure, clear gridded ruler, and French curve chief among them.
A flexible tape measure is the obvious one — body measurements, checking a hem is even, measuring a length of fabric — but on its own it's not enough for pattern work. A clear gridded ruler (the see-through plastic kind printed with a 1" grid) lets you true up a seam allowance, draw a line parallel to an existing edge, or check a right angle without a separate set square, and being able to see the fabric or pattern through it is the entire point.
The French curve and the hip curve are the two shaped rulers I use constantly and didn't understand the point of until I actually needed them: a French curve's tight, varying curve is built for armholes and necklines, where a hip curve's longer, gentler arc is built for exactly what it sounds like — hip and side-seam curves that are too broad for a French curve to trace smoothly. Trying to draft either of those curves freehand, or with a compass, almost never comes out as smooth as tracing along the right tool.
None of this replaces careful, consistent measuring technique — a tape measure held too loosely or too tight will give a different number every time, and that inconsistency causes more fit problems than any tool choice does.