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.
The scan is quite fast, the display human understandable so you can identify space-hogs, no matter how deep in your folder structure in seconds.
It's free and open source, so give it at try you wont be disappointed.