WinDirStat: Disk Usage Statistics Viewer

WinDirStat (Windows Directory Statistics) is a disk usage statistics viewer and cleanup tool for Windows.

On start up, WinDirStat 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.

Download:
WinDirStat