LibraryThing is a neat online service for cataloging your library (free for up to 200 books). It delves into social networking territory, allowing you to create a profile and connecting you to others who have similar libraries. It even offers book suggestions based on your catalog.
When Julia showed me this site, I kicked my heels with joy (figuratively speaking) - this is exactly the kind of thing the Inter-web is for! In a more civilized world, people would connect with like-minded peers using a site such as this, instead of trolling for jailbait on myspace at 3:00 AM.