Library Thing is a free cataloging tool for personal libraries. It actually does a very comprehensive job, complete with LC Classification, ISBN information, Publication information, Citation information, Comments and yes, my favorite, Tags.
An informative feature of Library Things is their tag clouds. They have three forms: Tag Cloud, Author Cloud and Cloud Mirror.
A tag cloud is simple and straight-forward. It basically shows how I have categorized my books and which category is most popular (indicated by large text).
An author cloud follows the same basic principle, it is a graphic list of the authors I have in my collection.
And finally, and most interesting, is the Mirror tag. The mirror tag shows my tags in relation to their popularity across the entire Library Things database. In other words, how many people have books that they deliberately tag with the same description.
All in all, my favorite application has to be Libary Things. It is an incredible resource to catalog and organize my library and make it a manageable set, where I can then gather my information effectively.