BleepingComputer Review:

WinDirStat scans one or more hard drives and produces a detailed map or the disk making space hogs immediately apparent.

It does this by drawing a rectangle for each file, the relative sizes being decided by the relative sizes of the files. So, a very big file will be a very big box and a small file would be a small box.

Product Description from Oliver Schneider:

WinDirStat is a disk usage statistics viewer and cleanup tool for various versions of Microsoft Windows.
 

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.