WinDirStat (http://windirstat.info)

Languages: 12 different languages, 4 with helpfiles. http://windirstat.info/translations.php
Compatibility: Windows 95 > Windows 7.
License: GNU General Public License / zlib/libpng license

Author's description:
WinDirStat (WDS) is a disk usage statistics viewer and cleanup tool for Windows. It shows disk, file and directory sizes in a treelist as well as graphically in a treemap, much like KDirStat or SequoiaView.

User's opinion:
I personally use this tool on every single computer I own, it's fantastic for visualising how much space is being used on a hard disk and what to delete in order to keep hard drive use to a minimum.

WinDirStat is already in LiberKey