I think I’m behind the curve on this, but recently I’ve been having fun with LibraryThing. You use it to make a catalog of your collection of books that you can access anywhere - why you’d need that I don’t know - but the cool part is you can browse other people’s collections that are similar to yours. You can also tag your books, explore books other people have tagged with the same terms, and get recommendations based on those tags. Very Web 2.0. I’m inputting books that I’ve read regardless of whether I own them, since having read the book seems the relevant bit of information, not whether I possess the physical object.
There’s some cool stuff going on in the background. To get populate its database of books, it uses web services to access information that Amazon.com and libraries make publicly available. It’s great that that data is employed in something cool.
(As an added bonus, LibraryThing is now also powering the “Recent Books” widget in the right-hand column. That’s much easier than my old method of hand html coding.)