In God of Light, by Playmous, you become a glowing ball of light named Shiny who lives in a darkening universe. And for a limited time you'll be able to download it for free.
You must conquer a variety of physics based puzzles to manipulate light and save the universe. The game has a pretty graphics and a moody soundtrack created by the band UNKLE. We thought it was pretty fun, actually. You might want to check it out while you have the chance to download it without having to pay.
We don't know exactly when God of Light will stop being free, but for the time being that's how much you'll find it going for on the App Store.