Hemisphere Games is giving away free promo codes for their great iPhone game, Osmos. To get your hands on a code, join the Osmos Facebook page, then wait until next week when they start posting batches of codes to the page every few hours. With batches of codes being posted every few hours each day, there should be plenty of codes to go around.
Unfortunately for international readers, the codes are only for US users. Check out this guide on how to redeem codes on the iTunes store if you are unfamiliar with the process.