This is neat (via Martin Kramer): LibraryThing

It's basically your own virtual bookshelf. You register an account (nothing more to do than creating a username and password -- not even an email required), then you start plugging your books in. Not only does it save your books for you, complete with cover graphics if available, but you can see who else on the service has the same books you have -- kind of neat for those rare books you figured you were the only person to own. If you have a lot of books it's pretty time-consuming to put your list in -- I'm only about 15% through my shelves and I hope I don't lose interest before I finish, but I think it will be useful once I'm done. My shelf so far.