If you have an iPhone or iPod touch I REALLY recommend "Rage" for iOS devices, but mainly for if you have 4th gen iPhone or iPod Touch as they have a gyroscope in their internals that allow you to play in "virtual window" mode.
That's where the gyroscope effectively makes the entire device like a REALLY GOOD pointing device, as good as a mouse to aim and FUN to use! Really, really, REALLY good game on iOS.