SpaceSniffer is a clever little freeware program which scans your hard disk and gives you a graphical representation of what folders and files are taking up what amount of space. It’s similar to the venerable Treesize in this regard, but offers a more in your face interface which is actually quite usable. The scan isn’t as fast as Treesize, and the file details can get a little cluttered, but overall I think this is probably a keeper insofar as it offers a nice overview of my disk on demand.
SpaceSniffer is a freeWare (donations are welcome) and portable tool application that lets you understand how folders and files are structured on your disks. By using a Treemap visualization layout, you have immediate perception of where big folders and files are placed on your devices. Treemap concept was invented by Prof. Ben Shneiderman, who kindly permitted the use of his concept into this tool.
There exists a similar tool called WinDirStat for windows and it's Linux counterpart, KDirStat, that does basically the same thing. I have been using it for quite some time but am going to try out SpaceSniffer.
Another reference, and this is 7 years old and my tool of choice since then: http://en.wikipedia.org/wiki/SequoiaView