Library Thing is a great took for those people who want to keep a good quality record of their personal library. It allows you to add tags (subject headings). I can see this a being a very helpful tool. Personally I don't think this is something I want to maintain.
There are also many other features, many of which are social. If you want to chat with other readers this is a great place to start. I do like the recommendations tab. Here you will find recommendations based on the books in your catalog.