If the puzzlers, platformers, and similar iPhone and iPod Touch games on the Apple App Store aren't to your liking, you might want to take a gander at Underworlds, an action RPG from Pixel Mine Games that just got released.
Underworlds is a Diablo-like dungeon crawler with thousands of unique armor, weapons, items, and treasures. The usual RPG fare - quests, leveling up, and stat upgrading - is also present. There's also a random dungeon generator so you don't play through the exact same dungeon every time.
Underworlds is already on the App Store and can be purchased for US 4.99. If you're still not too sure about it, check out this gameplay video: