GrandPerspective and Disk Inventory X are two free Mac OS X apps that give graphical views of where your disk space is being used. Both of these are based on work developed at the University of Maryland called Tree Maps. There are references at that link to many other implementations and similar projects on Windows and Linux.
When you want to see disk usage, a graphical view is intuitive and easy to understand. For example, here's how GrandPerspective sees my home folder:

GrandPerspective is very simple: the larger the rectangle, the larger the file it is representing. You click on the rectangle to see the path and name of the file. The only options available are to change the colors to have them selected by depth, directory, name or extension. That's it: this is a simple and very direct application.
Disk Inventory X is more ambitious. Like GrandPerspective, it displays rectangles based on the sizes of files. But it also shows much more information:
All of the gold rectangles in that image are Parallels Workstation documents (the same files are the large blue boxes in the GrandPerspective view). Notice to the right that documents of that type consume a total of 14.2GB of my hard drive. I can also see that I have 20.5MB of PDF files and so on. That's helpful information.
I can't really say that one of these is better than the other: it depends on what I'm looking for. The simplicity of GrandPerspective is attractive for a quick overview, but Disk Inventory X includes information GrandPerspective doesn't have. I'll keep them both.
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Mon May 8 22:21:17 2006: Subject: drag
For Linux-based stuff I like to use a KDE application called Filelight for this sort of thing.
http://www.kde-apps.org/content/show.php?content=9887
I beleive there are a few block-style file viewers like the ones you posted, but I find personally the circle view that filelight uses is much easier to read/use.
And on even more of a side note, you OS X users may have KDE4 environment to look forward to. http://wiki.kde.org/tiki-index.php?page=KDE/Mac
There were efforts to port KDE3 to 'native' OS X, and they got konqueror and parts of koffice working, but KDE3 just didn't work well outside of X. Qt4 libraries, which provide a break from the Qt3 API, should make it easier for KDE4 (currently vaporware) to work on Windows and OS X.
This is nice since KDE has a lot to offer.
Wed May 10 21:02:11 2006: Subject: TonyLawrence
And if you are a Linux user who wants a Mac-like desktop, see http://www.linuxplanet.com/linuxplanet/tutorials/6223/1/
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