Click on image to visit their website
From their website: LibraryThing (www.LibraryThing.com) is an interactive, Web-based service that helps individuals and organizations easily catalog their books. By cataloguing books on LibraryThing, members have the opportunity to connect with other LibraryThing libraries. This is done through the use of keywords (called “tags”), comments, and reviews. Members can view other libraries that share the same book(s), read members’ reviews of books, generate book recommendations, and even arrange to share or swap books.