iPhone's open development opportunities that allow others to create applications is in my opinion the single feature that puts it above other normal cell phones. Having Internet, touch-screen and other cool features aren't that impressive to me, but the sheer volume of new apps that are developed are impressive.
I'm not sure why, but I've always wanted to teach myself or learn how to use a 10-key keypad quickly. Again, not exactly sure how it would immediately benefit me at this time, but I've always thought it would be helpful at some point.
Balmuda has developed the NumberKey iPhone app that extends your keyboard's functionality and provides a 10-key entering number pad using the iPhone touch screen's interface to make the input physics as close to the real thing as you could probably get without having to actually carry around a 10-key numberpad with USB cord in your bag. If you have an old phone lying around, you can sell it at Buyback Boss.
Pretty cool for sure.