WinDirStat - Free Windows Disk Usage Tool

WinDirStat is a free and open source disk usage statistics viewer and cleanup tool for Microsoft Windows. This tool is extremely simple to use.

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.

More information about WinDirStat is available here.