This is Broken - A tribute to Seth Godin's book recommendations
Goodreads - See what your friends are reading.
Booknshelf - Social book and bookshelf discovery platform.
Readernaut - We are a small community of readers, sharing our adventures and supporting each other as we persue a life of learning.
Kooba - An interactive graph for finding new books
LibraryThing - A home for your books.