WinDirStat is a disk usage statistics viewer and cleanup tool for various versions of Microsoft Windows.
Note: if you are looking for an alternative for Linux, you are looking for KDirStat (apt-get install kdirstat on Debian-derivatives) and for MacOS X it would be Disk Inventory X or GrandPerspective.

Please visit the WinDirStat blog for more up-to-date information about the program.

On start up, it reads the whole directory tree once and then presents it in three useful views:

• The directory list, which resembles the tree view of the Windows Explorer but is sorted by file/subtree size,
• The treemap, which shows the whole contents of the directory tree straight away,
• The extension list, which serves as a legend and shows statistics about the file types.

The treemap represents each file as a colored rectangle, the area of which is proportional to the file‘s size. The rectangles are arranged in such a way, that directories again make up rectangles, which contain all their files and subdirectories. So their area is proportional to the size of the subtrees. The color of a rectangle indicates the type of the file, as shown in the extension list. The cushion shading additionally brings out the directory structure

https://windirstat.info/