The Fling controller from TenOne Design (soon to be reviewed) is a great way to add a physical to your iPhone or iPad, just by suction-cupping it onto the screen. This means that it works with any game on your iOS device that uses an on-screen “joystick.”
The downside is that it moves at the worst moments: I have wiped out in more than one GTA car chase this way. But designers at the Keio University in Japan have come up with another idea. A joystick which uses the iPhone’s camera as a controller.