Goodreads - See what your friends are reading.
LibraryThing - A home for your books.
Delicious Library - Delicious Library 3: Catalog your books, movies, music, software, toys, tools, electronics, & video games. Archive your life.
BookAuthority - BookAuthority collects the most recommended books on business, technology and science - as featured on CNN, Inc and Forbes
GoodBooks.io - Largest curated collection of 8,500+ book recommendations.
Bookshelf - Your virtual library - Organize your home library books on multiple virtual bookshelves.