LibraryThing is a cataloging and social networking site for book lovers.
LibraryThing helps you create a library-quality catalog of books: books you own, books you've read, books you'd like to read.
You can contribute tags, ratings and reviews for a book, and Common Knowledge (facts about a book or author, like character names and awards), participate in member forums, or join the Early Reviewers program.
LibraryThing is a cataloging and social networking site for book lovers.
You can also join genre specific book clubs!
And if you want your very own online catalog, you can create one using LibraryThing's TinyCat!
Goodreads is like Facebook for book lovers!
It includes:
Listopia: A collection of lists by subject organized by member votes. You can use it to find like books and also rate your favorite books.
You can add books to your currently-reading, to-read, and read shelf. Goodreads allows you to rate a book based on a 5-star system. You can also write a review for a book you have read.