Readernaut - We are a small community of readers, sharing our adventures and supporting each other as we persue a life of learning.
Goodreads - See what your friends are reading.
aNobii - aNobii is a social network for listing books, sharing insights, and meeting people.
LibraryThing - A home for your books.
Riffle - Just good books and the readers who love them.
Kooba - An interactive graph for finding new books