Goodreads - See what your friends are reading.
Tellico - Tellico is a KDE application for organizing your collections.
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.
What Should I Read Next? - Book recommendations
Readerware Book Database - The easiest, fastest way to catalog your books, nothing else comes close.