In readying the iPad 2 version of Two Worlds II: Castle Defense (released on PC, Mac, iPhone, and iPad back in May), TopWare has added a really cool new feature. Using the iPad 2's camera, Castle Defense can track the player's eyes and use that information as part of "3D-ET." This technology allows the player to move their eyes to look around corners in the newly modified game engine. As the viewer looks around, the perspective will shift.
It's a cool idea that has rarely been used in games so far, with the only recent example I can think of being a hidden picture DSiWare game. This technology probably won't be used much in the future because the iPad 2's gyroscope can probably do the effect easier, but I still think it's a neat way of using a camera. You can check out the effect for free by downloading the Lite version of the game, or you can drop money on the full version.