LibraryThing.com lifetime account [$25]
Tim Spalding, the creator of LibraryThing, has unleashed the inner librarian of thousands of users. LibraryThing tracks your books, and lets you tag, rate, and review them; it lets you see others who own that book, people who use similar tags, plus a hundred other features. LibraryThing taps the catalogs of Amazon (all the different flavors), the Library of Congress, and more than 40 world libraries in order to help identify books and open up cataloging data. If you’re the sort of person who compulsively checks out other people’s bookshelves, you can’t miss LibraryThing.