GrandPerspective

The tool on this list we've probably been using the longest (well over 20 years), GrandPerspective hasn't changed at all and is still as functional as ever. Essentially it scans any hard drive and breaks its contents down into little blocks, sorted by type (in different colors) and then represented, to scale, by size.

We most often use it to free up space on drives by identifying large items/files/folders but we can't tell you how many times running a scan has yielded surprising results. You won't use it too often, but when you need it — nothing else will do.