Readerware Book Database - The easiest, fastest way to catalog your books, nothing else comes close.
LibraryThing - A home for your books.
Tellico - Tellico is a KDE application for organizing your collections.
Delicious Library - Delicious Library 3: Catalog your books, movies, music, software, toys, tools, electronics, & video games. Archive your life.
Book Catalogue - This is a simple book catalogue application, to store a list of your books.
Goodreads - See what your friends are reading.