Bookrabbit is an online bookshop, but with social networking thrown in also. Catalogue your own books, create your own categories, find friends, read other catalogues, join a book group. Now allows imports from LibraryThing and Shelfari.
Librarything allows you to catalogue your books, using amazon searches, the Libary of Congress or other world libaries. Review, and rate these, and share with friends. Import from anywhere. Tag, list by book cover or lists, there are large discussion groups also, an early reviewers club, and LibraryThing widgets to share your book collection in various ways on your blog. Free accounts allow for 200 books. Over this, a small subscription yearly or a lifetime membership is available.
Shelfari is a social network for people who love books. Create a virtual shelf to show off your books (which looks like a shelf), share them with friends. Pretty site.
Good Reads is another social network on books, with quite a good author program.
What's on my Bookshelf offers cataloguing, tags, popular tags, and users can trade books using a points system
Books Well Read offer cataloguing facilities, sharing, but private comments on books (rather than public reviews) if the user just wants to make notes on their books. You can also add multiple entries per book. If public, these can be classified as spoilers.
Revish offers shared reading lists, reviews, book groups, widgets.
BookMooch - exchange books.
Frugal Reader allows users to trade books for free, discuss them and other features.
Bibliophil.org has a lot of features including cataloguing, friends, sorting by many fields, ratings of books, recommendations, loaning of books, search for best price, wish lists, data export, mobile access, and it's all free.
Book Tribes provides more book lists, groups, sharing and tags.
Connect Via Books is a London based social group. Enter the title or author of a book you have read, and you will be shown a list of other people who have read the same book.
Listal not only does books, but music, movies, games and more. Tag them, and share with a community.
Mediachest allows you to catalogue, track and borrow books, cds, dvds and games. Good groups features - use it for families, friends and co-worker groups for your exchanges.
Reader Squared - catalgue, tag, find friends, discuss books and authors.
Squirl beta - catalogue books, stamps, coins, comic books, records, join groups
Shelf Centred allows you to catalogue movies, books, video games via ISBN or keyword search. Organise in a shelf, make wishlists, and find friends. You can't see these lists until you've registered up (free)
Book Jetty allows you to catalog, rate and review books, check availability through libraries (from 11 different countries), discuss books with firends, display on your blog
Juicespot allows people to comment on books, review them, and recommend.