LibraryThing is the coolest Thing ever. Long have I longed for a card-catalog of my personal library (ok, you already knew I was a bookwormy geek). Now comes LibraryThing.com, which has quickly become my favorite new time sink. You make up a user name and start building your bookshelf. All you do is enter the title or the ISBN (I like the ISBN's b/c I can do 'em fast), and LibraryThing links to Amazon & the Library of Congress to fill in all the rest for you. It even puts up pictures of the covers. I did 6 of my bookshelves in and evening- only 3 shelves to go!
Then comes the extra fun part: LibraryThing does lots of stuff with your book list. You can rate your books, tag them with notes and keywords, and write reviews. LibraryThing shows you how many other Thing-ers have the same book as you, and how many of those have the same edition. There are discussion circles for titles, authors, genres, and subjects, and you can start a new one anytime. Then there are the recommendations. Oh, never have I seen such spot-on recommendations! You have to check this Thing out.
By the way, if you want to see my book collection, it's here.