WinDirStat is a disk usage statistics viewer and cleanup tool for Microsoft Windows. On start up, WinDirStats reads the whole directory tree once and then presents it in three useful views:
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.