This is a great little utility that Brian showed me.
Tonight, my wife Candy was trying to print something, and had to download a new print driver. Suddenly, her hard drive was reporting full, and there's no way that should be. We couldn't figure out why, so we downloaded Grand Perspective. Grand Perspective scans a folder (up to the whole hard drive) and shows a graphical representation of how much space each file occupies, like this:
We pointed it to her root directory, it trundled away for a few minutes, and made it easy to spot the culprit: an aborted attempt to add a whole bunch of images to iPhoto that had copied them locally. Delete them, and 65+ Gb free again.
Highly special purpose but, if you need it, you need it badly. I'm about to use it to trim my Subversion repository to something a little more manageable.