There are visual tools, such as DiskInventoryX, which divides the used space on the hard drive into different compartments, each one in a different color.
WhatSize, however, does the same, but numerically. That is, it displays the used space by file, folder, and subfolder in bytes.
The program analyzes whichever unit of the hard drive that the user selects and organizes its content in a descending list according to the amount of space its content takes up. This can also be done with just a single folder, without having to completely analyze the hard drive.
NotesFor Mac OS X 10.3.9 or later. The program can be used freely, although it is limited to 20 GB.