GrandPerspective is a small utility application for Mac OS X that graphically shows the disk usage within a file system. It can help you to manage your disk, as you can easily spot which files and folders take up the most space. It uses a so-called tree map for visualisation. Each file is shown as a rectangle with an area proportional to the file's size. Files in the same folder appear together, but their placement is otherwise arbitrary.
FROMEOUT11 Fantastic program for clearing out your hard drive, or finding out what's taking up all of your disk space. Does exactly as described, with no stability issues for me thus far. (Version 0.9.11)
NADEEMF I find tree maps the absolute best way of representing disk usage, and I think there is no better program to do it on any platform. (Version 0.9.11)