Goodreads - See what your friends are reading.
What Should I Read Next? - Book recommendations
LibraryThing - A home for your books.
BooksReddit - BooksReddit is a social book recommendation aggregator.
BookAuthority - BookAuthority collects the most recommended books on business, technology and science - as featured on CNN, Inc and Forbes
BookSloth - Book Sloth lets you add books, post and see reviews as well as interact with other readers.