deborphan finds "orphaned" packages on your system. It determines which packages have no other packages depending on their installation, and shows you a list of these packages. It is most useful when finding libraries, but it can be used on packages in all sections.
If you've had a Debian box running for a while that's had a lot of packages installed and removed, deborphan is a handy tool for finding some of the cruft1 that's accumulated on your system. It's especially handy at giving you a list of libraries that are no longer required (assuming packages have the appropriate dependencies documented).
More information on this package can be found on the Debian web site.
(If there is a package you would like to see featured here, go to the userinfo page and follow the directions there to submit your entry.)
1Yes, cruft. Would anyone care to write up a short comparison of the two to help folks choose which one is right for their needs.