WinDirStat- http://windirstat.sourceforge.net/

WinDirStat is a disk usage statistics viewer and cleanup tool for Microsoft Windows (all current variants).
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.