WinDirStat is a free and open-source graphical disk usage analyzer for 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.
License: Free software published under the GNU General Public License, version 2.
Link : WinDirStat