GrandPerspective is an awesome utility that will that will graphically shows the disk usage within your MAC OS X 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 visualization. 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.
What does your hard drive look like?
