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; and the extension list, which serves as a legend and shows statistics about the file types.