LibraryThing is an enormous lot of fun to play with. Not only is there finally an entirely workable web-based tool for cataloging your personal library--a tool changing and improving right before our eyes with the work of programmer Tim Spalding, and an already large community of users-- but it comes with the pleasures of social networking, if you care to play. It will show you how many librarything users also have your book, how many others of your books you share with each of those people, what other books they have, what 'tags' (subject headings or sorting terms) they used, what reviews they have posted... you can communicate with each other.
You may enter up to 200 books for free. To explore a little, go to the zeitgeist page. You may get addicted to looking at people's tag clouds and author clouds. The most-owned books among the 26,472 users are the Harry Potters, The Hobbit, The Da Vinci Code, and Pride and Prejudice. ??Pride and Prejudice?? Read some of the kudos on the buzz page.
The phrase above, "what we've got in common," comes from one of the conversations on the librarything blog.