This list started out on the excellent Reader's Advisory listserv
FictionL. I have added to and annotated it. These social networking/reader-sharing sites may prove helpful when directing patrons to books or for trying to decide "what to read next."
BiblioTravel: A free online resource for identifying stories that are set in distinct locales started by two librarians.
Book Blogs: “Our members read books, blog books, write books, and publicize books.”
Book Crossing: Where books take on a life of their own. BookCrossing is earth-friendly, and gives you a way to share your books, clear your shelves, and conserve precious resources at the same time. A book registered on BookCrossing is ready for adventure.
Book Glutton: Our intent: build an experience that is simultaneously a book group, a computer, and a book. Chat chapter by chapter, comment on each paragraph.
Book Jetty: BookJetty is a social utility that connects you with your friends' bookshelves and checks books' availability in the libraries.
Book Movement: “BookMovement.com was founded in 2001 to give book clubs a way to recommend books to each other on a national level.”
Books I Read: A Facebook application. (Must register to use.)
Books Well Read: The goal of BooksWellRead is to be the online destination of choice for people who want to 'digest' what they read by capturing their thoughts in writing. BooksWellRead has been designed to be simple, fast, and easy to use.
Crime Space: A place for readers and writers of crime fiction to meet.
eBooklists: RA wiki being constructed by librarians who were in graduate school together.
Goodreads: Keeps track of what you’ve read and what your friends are reading. Also on Facebook.
I Read Wiki: A collaborative website by Iowa librarians of reading suggestions.
Library Thing: Helps you create a library-quality catalog of your books and connects you with people who read the same things.
Reader2Reader: A UK network that allows readers to discover and suggest books to each other, chat and share reader experiences or do author/title searchers.
Read Me: User editable wiki of book suggestions and queries.
Revish: Revish is a place where people share their reading experiences. You can use Revish to keep and share a list of books you're reading, write reviews of books, find recommendations from other readers, and a whole lot more.
Shelfari: Allows you to share what is on your bookshelf with others and discover new books. Also on Facebook.
What Should I Read Next?: Searchable database of real readers’ favorite books (over 47,000 and growing) to suggest what you could read next.
What's On My Bookshelf: This is a book trading communitywhich allows members to exchange their books using a simple point system.
Which Book: This is a book recommendation tool that makes suggestions based on your moods.