http://www.librarything.com/
Excerpt:
What is LibraryThing?
Enter what you're reading or your whole library—it's an easy, library-quality catalog. LibraryThing also connects you with people who read the same things.
Go ahead and take the tour.
What's good?
* Meet the world's largest book club. Find people with eerily similar tastes.
* Catalog with Amazon, the Library of Congress or 78 other world libraries. Import from anywhere.
* Get recommendations. Tag your books and explore others' tags.
* Put your books on your blog.
* Enter 200 books for free, as many as you like for $10 (year) or $25 (life).
It seems to me that this would be a good thing for those seeking renters' or homeowners' insurance too.