Goodreads - See what your friends are reading.
Readerware Book Database - The easiest, fastest way to catalog your books, nothing else comes close.
GoodBooks.io - Largest curated collection of 8,500+ book recommendations.
Tellico - Tellico is a KDE application for organizing your collections.
Bookicious - Find the best new book to read with books collections for makers, founders and entrepreneurs.
Book Catalogue - This is a simple book catalogue application, to store a list of your books.