I've been having a great time building my online library at LibraryThing. It's a service that allows you to catalog the books that you own online. I'm not sure yet if it's really useful, but it's fun. You can compare the books you own to books that other people own, and view similar libraries. It's quite addictive. It's free for up to 200 books. If you want to catalog more than that, it currently costs $10--a lifetime fee. I only have 100 books up there now, but I just bought a paid subscription as a hedge in case that very reasonable fee goes up in the future.