LibraryThing

What should I read next? What are some great YA fantasy books? I've read all the Alex Rider books, what else is good?
Join LibraryThing, and you'll be connected with thousands of book lovers and all their ideas.
You start off by cataloguing your own books online. This is easy and fast, and much cleverer than it appears. Under its bonnet, LibraryThing uses the power of z39.50 searching to interrogate the world's major library catalogues and find professionally catalogued details of the book you are referring to.

Once you've catalogued a few books the fun begins. You can check out who else has catalogued similar books, browse their collections and see what you might like to read next. Say you had a very specific interest in vampire books that none of your friends understood. All of a sudden, on LibraryThing you can find a group of like-minded people, check out their collections, add your book review to theirs, and form or join a group on the topic.

One popular group is the "50 book challenge". Members aim to read 50 books in a year, and post information on what they have read so far and what they think of it. Other people comment on their choices and make suggestions about what they might like to read next. There are some great conversations going on on this group.
Other popular groups include Children's Fiction, and Read YA Lit, which choses one book a month to have a discussion about. March's book is The Giver by Lois Lowry. They also have great "group talk" discussions about a wide variety of other topics.
LibraryThing is very popular with librarians. In fact, the group Librarians who LibraryThing is the most popular, with 3655 members.

Or, you could just use LibraryThing’s powerful search system to find recommended books. Look up Eragon by Christopher Paolini, for example, and LibraryThing will show you the most popular books that people with Eragon also have in their library. Click on more recommendations for specifics. This type of search can pull up interesting results, as its not limited to any genre or type of book, just to popularity by lots of Eragon readers. People with Eregon also seem to buy Inkspell by Cornelia Funke, and Magyk by Angie Sage. The recommendations work best with popular books – have a look at the top of a book’s entry to see how many members have this book (5,332 for Eragon).

Here's a good link to an article about LibraryThing.