Library Thing

Library Thing allows you to catalog your home collection of books. Because you put the catalog online, you can connect and share with users who have the same interest/books. Plus it has the easiest registration page ever. If you're not quite obsessive enough to catalog your personal collection, you can still use Library Thing to browse and find book recommendations based on other people's collections.

"Your profile connects you to people who share your books. With over 79,000 users and 5.6 million books in the system, you'll find some 'eerily similar' libraries."

Here's their intro tour:
http://www.librarything.com/tour/1