Goodreads - See what your friends are reading.
LibraryThing - A home for your books.
Riffle - Just good books and the readers who love them.
Delicious Library - Delicious Library 3: Catalog your books, movies, music, software, toys, tools, electronics, & video games. Archive your life.
Readernaut - We are a small community of readers, sharing our adventures and supporting each other as we persue a life of learning.
Bookshelf - Your virtual library - Organize your home library books on multiple virtual bookshelves.