One of my favorite apps in the early days of the App Store, SnapTell Explorer, has recently updated to version 2.0, and while they've dropped the "Explorer" part and were purchased by Amazon earlier this year, the app still offers the same impressive functionality: take a shot of a book, DVD, or album with the iPhone, and have it pull up ratings, information, and prices on the item in question. Despite the Amazon buyout, it still offers prices from elsewhere, though the Amazon mobile store in the app is the best-looking choice. The app now also lets you share "snaps" (you can email a found item to a friend for, say, a holiday wishlist), and it has a few reporting options for incorrect matches, to make their system even better.
I'm still amazed by this app and how it can pull up an object from almost any picture -- more than a few times I've been in a bookstore or music store, and pulled up the app to snap an item, only to find it cheaper somewhere online. The app is a free download, too -- I can't think of a more must-have app for serious comparison shoppers.