Owning an iPhone means opening the door to a whole host of addictions — sweet, wonderful addictions. Sure, there’s the hardware to get hooked on: part phone, part music player, part portable theater, all wrapped in the most elegant interface ever to grace a handset (okay, so that hurdle wasn’t so high). But then there are the apps, all those easy-to-download, seamlessly installed, usually cheap add-on programs available at Apple’s App Store (which recently marked its 1-billionth download). Apps are an extraordinarily powerful way to customize an iPhone (or iPod Touch — they both use the same operating system) and get it to do just the things you want it to. Here are some of our favorite apps, some useful, some just plain fun.

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