"Goodreads is the largest social network for readers in the world. We have more than 3,200,000 members who have added more than 84,000,000 books to their shelves. A place for casual readers and bona-fide bookworms alike, Goodreads members recommend books, compare what they are reading, keep track of what they've read and would like to read, form book clubs and much more."
"LibraryThing is an online service to help people catalog their books easily. You can access your catalog from anywhere—even on your mobile phone. Because everyone catalogs together, LibraryThing also connects people with the same books, comes up with suggestions for what to read next, and so forth."
Open Library's goal is to create a page of information for every book ever published. Visitors to the site can find information on over 20 million books, use resources for finding books online, browse the books on a variety of subjects, and more!