Librarything

If you haven't already, go check out Librarything. You can put your library inventory on-line. The benefits of this are many: first, you actually produce an inventory of your books. I've rediscovered stuff I'd forgotten I had, and also realized that I don't have some books that I thought I did (and am pretty certain that I used to). Second, I now have a list of books for my insurance company to add to my household inventory. And trust me, this is not a trivial benefit. Some of the other advantages are being able with a click to see all the works by a certain author, being able to see the various editions of a given work, and just the fun of browsing through other people's libraries.

You can take a look at mine here.

EDIT: check out A very short introduction to LibraryThing. It tells you 'most everything you'd want to know before you sign-up, and then you can sign-up!