WinDirStat

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.

Website: http://windirstat.info