WinDirStat

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

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.