This one took me a little bit to wrap my head around. Flypad, available on iTunes, allows you to control PC racing games like Need For Speed: The Run and DiRT 3 with touchscreen and tilt controls by downloading the iOS app as well as the desktop client.
Motion controls have yet to really take off on PC despite efforts by Razer and their Hydra so this is definitely a neat approach, using your iOS device to interact with a PC game. More interesting than the idea itself would be the number of PC gamers interested in utilizing this method.
Tip: You can swipe photos or use your arrow keys