I just found this program OverDisk
, and I urge you all to check it out.
It scans your hard-drive (~30s for the 85k files on my main HDD) and maps your files/folders into a sort of multi-leveled pie-chart, starting from the center at C: (or D:, etc. depending on which drive is selected) and shows the proportions of space taken. Clicking on a subfolder "re-centers", showing the size distribution of its children.
The best way to understand what I'm trying to say is to try it yourself!
I just used it to find 7GB of Protected Files left over after uninstalling Norton. I could also see it as being very useful to determine which directories to back up or even better in the all-important spring cleaning!