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 or apt-get install k4dirstat on Debian-derivatives) or QDirStat and for MacOS X it would be Disk Inventory X or GrandPerspective.
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.