Osmos HD is a physics based puzzler that costs $2.99 and has zero in app purchases.
Pro Multiple game modes to choose from
There are two game modes for Osmos to choose from: Odyssey and Arcade. Odyssey is a 27-level long campaign mode that takes you through the story of Osmos, all the while progressing in complexity. On the other hand, Arcade is a free-play mode with adjustable difficulty and game types that are unlocked once Odyssey mode has been completed.