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.
inventaire.io - Keep an inventory of your books. Share it with others. Discover the books available in your network!
Goodreads - See what your friends are reading.
Palatina - Palatina is a Ruby library for managing publication collections.