MS Research has been working on an interesting thing: .NET components that display hierarchical data as a set of nested boxes. For questions or comments, go to the microsoft.public.research.netscan.discussion newsgroup.
It seems that this tool was used first to do a newsgroups survey and analyze their importance in cyberspace.
The site C2i give an example on how to use TreeMap to show the space files are using on an hard drive.