Where did all my hard drive space go?

A recent thread on the mailing list focused on tools to get back some space eaten up by files and programs and who knows what. With permission, I post the cherry on the cake:
Ed, thanks for the most beneficial program I've ever downloaded. GrandPerspective enabled me to see where all my memory was being wasted. I used it and went from 8.2 gigs to almost 21 gigs of free HD space. Doc
I'd say that's a pretty good endorsement of GrandPerspective, a free (donations accepted) program that simply does one thing really well.