GrandPerspective is a neat little program for OS X that shows disk usage in an intuitive way; I somehow managed to find this during my random wanderings of teh [sic] intarwebs. From the project website:

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.

If you’re running OS X, I would definitely give it a try! It is free, after all. :)