Song Summoner was one of the few games released for the iPod—not the touch, but original clickwheel type. It was special, as a game, because it used your own music to procedurally generate enemies. Now, it's on the iPhone.
It's $US10, but it's an RPG, which supposedly implies more gameplay hours than non-RPGs. Even when you're not actually playing the game, you can level up your guys by listening to the music that's bound to each character, giving you an added bonus to use your iPhone/iTouch more. [iTunes (Full Version) and iTUnes (Lite) via Kotaku]