WinDirStat is a disk usage statistics viewer and cleanup tool, inspired by KDirStat .
TreeSize Professional tells you where precious disk space has gone to. It can be started from the context menu of a local folder or a local drive and displays the size of each folder, including its subfolders.
GrandPerspective is a small utility application for Mac 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.
SquirrelDisk shows you exactly what's taking up all that space and helps you clear it out. Using SquirrelDisk is a matter of clicking one button to easily find all the huge files on your computer.
Create a visual representation of the amount of space each file is taking on your hard drive. It creates a "squarified treemap" that makes it easy to see where all your space is going.Discontinued
The official website is no longer available. Last version, 1.3, released in June 2006, can be still downloaded from Softpedia.
DiskWave is a free utility to help you determine where most of your hard drive space is consumed.
The program seems to be dead, as announced in a tweet of the author: https://twitter.com/bartheph/status/742596462807265280 . The website and the changelog look abandoned. Last version, 0.4.3, released by the end of 2015, can be still downloaded from the official website.