Readerware Book Database - The easiest, fastest way to catalog your books, nothing else comes close.
Goodreads - See what your friends are reading.
Tellico - Tellico is a KDE application for organizing your collections.
LibraryThing - A home for your books.
Book Catalogue - This is a simple book catalogue application, to store a list of your books.
GoodBooks.io - Largest curated collection of 8,500+ book recommendations.