LibraryThing: Track your books and to whom they are out on loan; join the world's largest book club

A home for your books.

Enter what you’re reading or your whole library. It’s an easy, library-quality catalog.
A community of 1,700,000 book lovers.

LibraryThing connects you to people who read what you do.

And it is free.

http://www.librarything.com/home