GrandPerspective finds those giant files that suck up your disk space.

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 treemap 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.