aNobii  List Books, Share Thoughts, Meet People. The  name? From Anobium Punctatum, which is the proper name for bookworms.

Bibliophil

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.

BookMooch Give Books Away. Get Books You Want. Swap books, all you pay is postage.

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.

Chainreading

eBooklists  RAwiki being constructed by librarians who were in graduate school together at Indiana University.

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 ibrarians 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.

Listal

Paperback Swap Just like it sounds. Over 2 million paperbacks available for swapping.

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 allows you 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.

weRead is a community for book lovers.

What Should I Read Next?  Searchable database of real readers’ favourite books (over 47,000 and growing) to suggest what you could read next.

What’s On My Bookshelf, a book trading community, allows members to exchange their books using a simple point system. Users maintain their own profile page with friends, book inventory, and wishlists.