I’m really becoming addicted to aNobii, and I like for a couple of reasons.
I like it as a social tool, because among DVD, CD and Books, the last are better hit (or, they can benefit more) from the “social network effect”, i.e. from the exchange of user reviews, comments and whatever, so it’s easy to discover “users with similar bookshelfs”.
But I really like it also because it has one of the best user interfaces I’ve ever seen in a web application.
Just to give you an example, I’m posting a picture of the ISBN validation textbox. Very nice.