This colorful little app is GrandPerspective, my new favorite little bit of freeware for OS X. You can guess what it does. Give it a directory (which can be root level if you like) and it scans everything from there onwards, sizing every file it can find.
Then it displays this wonderful view of your files as little colored boxes; related files are grouped, and just by moving your pointer over them you can see from the text field below what’s what. All your music files appear as a mass of fairly small little squares. Big stuff, like swap files, is easy to spot.
As are space hogs. At the weekend, when a friend asked me why his 30GB disk was nearly full even though he had less than 4GB of music files, I suggested he use GrandPerspective to weed out the hogs. And guess what, after 30 minutes he’d identified nearly *8GB* of unwanted, unnecessary stuff that he could delete. Neat.