Mac memory allocation visualization tool?

[asterroc]

Somebody on my flist posted a program that would let me see where all my memory is being allocated on my Mac. The problem is that I've got only a couple Gigs of HD space left (used 89 Gigs of 93), and I just moved all my document files off onto an external HD, and that was only 2-4 Gigs. So I want to know if the rest are images, audio, programs I don't use, or what. Help!