Inertia is a really amazing platformer for the iPhone that uses a mechanic I haven't seen in any game in recent memory. It's a platformer, so you control a little spaceman that can walk left and right and jump from platform to platform. But Intertia adds another button that essentially will keep you moving at whatever speed and direction you're currently moving, sans gravity, when you press it. If that sounds confusing, it is, and fortunately, the game has quite a few levels of tutorials to really get you used to how the inertia button moves and plays. If you're rocketing up and you press the button, you'll keep rocketing up. If you're falling down at an angle, you'll keep falling, at exactly that angle.
Once you figure it out, it's quite fun. And that very original movement also opens up all kinds of cool levels, where your little spaceman bounces around through 2D environments collecting various scrap pieces and running time trials. There are 35 levels to go through, and the game uses OpenFeint (not Game Center, though) for leaderboards and achievements.
The lite version is well worth the download to check out the mechanic if nothing else, and you can buy a universal HD version for $3.99. I'm really impressed with this one -- the iPhone isn't always the best device to play a platformer on (given that movement really needs to be precise sometimes), but this inertia mechanic really opens up some new elements of a very old, well-traveled genre.