NOTE: I am not affiliated with this site (other than having signed up for a user account myself!); I just found it today via a reference in the Bookseller and thought it ought to be shared.
BookRabbit is a relatively new (launched mid-May) sort of social networking site for books - like LibraryThing, but flashier and with a transactional bit. The idea is, you take photos of your bookshelves, label the books, and then you can compare your shelves with other users.
At the moment, they have a promotion whereby they have 1000 free books to give away to new users who upload a bookshelf and tag at least five books in it. They will select a book based on the books you've tagged that they think will suit your taste. The promotion started on 30 June so if it's been popular there may not be all that many left, but you never know...
Interested to hear other people's views on this too..?