Apple's iPhone has revolutionized cell phones in more ways than can be counted. It introduced a gorgeous touch-screen interface -- that worked intuitively. It transformed voicemail form an analogue set of number prompts into something users could manipulate easily. And, of course, it created an entirely new market for lightweight software called apps. On the evening of March 3, Chunli Fu of Qingdao, China, downloaded a free version of Disney's Where's My Water? app. It marked the 25 billion download milestone. Now, here are the most popular iPhone apps -- ever.