Sometimes you just don't want to cart a bunch of crap around. That's why iPhone cases like this one from Case Mate come in handy -- they let you slip your plastic discreetly into the back.
It's got enough room for two cards, whether that's your Visa and your driver's licence or your library card and your train ticket, or whatever. It won't obstruct your access to your headphone jack, 30-pin dock, volume control, etc. It comes in black, silver, and pink, and it will cost you US$35. For me, it means ditching my purse, which I can't argue with. Now if only someone would make a case capable of holding house keys, too. [Case Mate via Real Simple]