Cube [iTunes Link] is an adaptation of Doom to the iPhone, developed by fernLigthening [iTunes Link]. The controls are amazing: accelerometer for sight, aim and direction (there are the same); tap the corners to move, shoot, pause and go to the menu). Surprisingly enough it is free (so far) but we can expect a paying version with more levels coming.
The 3D engine (a projected 2D-height map, actually) is really fluid and the motion-response is great; just check this video demo:
A few bad points: many do not like the very sensitive point-and-shoot: being able to tap on the screen to shoot on that particular direction might have been easier—but I prefer to see the screeen at all times, as fernLightening has made possible. One thing however: do not try to play it in a car or even a train: you won’t be able to aim, but more importantly, you will get sick. The motion-by-toggle has just the right setting that messes with your sense of balance.