'Library Thing'

"Library Thing pulls the information from Amazon, so you're bound to find your book in the search results. When you add a book to your library, tag it withkeywords. This helps you organize books by subject or location. I like to tag my books as read or unread. But my favorite feature is the reviews. The site has a lot of members, and many of them post comments on books." - Kim Komando (item spotted by David Faucheux).

Addendum from David Rothman: Also see Branko Collin's post to the main TeleRead blog.