WinDirStat

WinDirStat is a Windows cleanup tool and usage viewer. It is also released under GPL so it is open-source.
It is one of the most unique and disk usage viewers since it shows the tree map along with file sizes and is able to graphically represent the disk right away. This is useful in seeing what is on the disk and how it is layed out.
Sometimes, it’s nice to know how to use a drive effectively when installing a program or removing one.
Via WinDirStat