Disk Inventory X
Disk Inventory X is a disk usage utility for Mac OS X 10.3 (and later). It shows the sizes of files and folders in a special graphical way called "treemaps".
If you've ever wondered where all your disk space has gone, Disk Inventory X will help you to answer this question.
For a quick start please read the readme file on the disk image. Or see a 1 minute video about Disk Inventory X from macbreak.com.