WinDirStat (WDS) is an Open Source 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.
It gives a very powerful visual picture of the contents of your hard drive (using a graphical representation: tile charts).
WinDirStat reads the whole directory tree once and then presents it in three useful views:
1) The directory list, which resembles the tree view of the Windows Explorer but is sorted by file/subtree size,
2) The treemap, which shows the whole contents of the directory tree straight away,
3) The extension list, which serves as a legend and shows statistics about the file types.
Once you’ve found the offending rectangle that’s taking up all that HDD space, then right click and select the delete button to get your own back.
What is a Treemap?
The treemap represents each file as a coloured rectangle, the area of which is proportional to the file's size. The rectangles are arranged in such a way, that directories again make up rectangles, which contain all their files and subdirectories. So their area is proportional to the size of the subtrees. The colour of a rectangle indicates the type of the file, as shown in the extension list. The cushion shading additionally brings out the directory structure. Treemaps were invented by Ben Shneiderman (www.cs.umd.edu/hcil/treemap-history).
See here for more info: http://sourceforge.net/projects/windirstat/ or http://windirstat.info/.
For Linux (with KDE interface) there is the following equivalent: http://kdirstat.sourceforge.net/. This is actually the original on which windirstat is based.
For Linux (with Gnome interface) there is the following equivalent: http://www.marzocca.net/linux/baobab.html