DaisyDisk - where has all my disk space gone? (Mac app)
DaisyDisk is the best app I've found so far for visualising where all my disk space has gone. It's free, but you can pay $20 to get it to stop nagging you.
P.S. I have a theory that inability to manage one's finances and one's hard disk space have the same psychological factors at their root. ("Oh, I'll treat myself just this once...", "It's only a small thing", etc)