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, books you've lent out ... whatever grouping you'd like.
Since everyone catalogs online, they also catalog together. You can contribute tags, ratings and reviews for a book, and Common Knowledge (facts about a book or author, like character names and awards), as well as participate in member forums or join the Early Reviewers program. Everyone gets the benefit of everyone else's work. LibraryThing connects people based on the books they share.
I will be signing up.