This is for Windows, but despite that shortcoming, it's pretty neat! It would be cool if someone integrated this into xdiskusage...
It works *great.* We have some Windows machines in the office that multiple people use, and their hard drives were mysteriously full. I found the offending files (no shit, some dork's pdfs of Werewolf: The Apocalypse books) in nothing flat.
http://www.sweetcode.org (syndicated to <lj user=sweetcode>) often reports interesting software like this.
Very nice. Beats Diskmapper with a stick.
Ever seen scanner?
Here are a couple, free, for OSX: