Amazon's usually the first place I look during a fit of impulse buying, which their iPhone app now makes stupidly easy.
Product searches, comparisons and account management are a given, but what really pushes this one over the edge is a new feature called "Remembers." Just snap a picture of a mysterious product or thing, and Amazon will get back to you with a surprisingly accurate, impressively quick suggestion as to what it is. Then it will sell said thing to you. Magic.
From the App Developers: The Amazon Mobile app allows iPhone and iPod touch owners to very quickly search, shop, compare prices, read reviews, and make purchases on Amazon.com using a simple, yet elegant, interface. Amazon customers have full access to their existing cart, wish lists, payment and shipping options, order history, 1-Click (R) settings, and Prime membership benefits.