Osmos, the PC, Mac, and Linux game from Hemisphere Games, has been in development for a long time for the iOS, initially for the iPhone and iPod touch, but its first release was for the iPad. Now the game is available on the platforms it was originally designed for, the iPhone and iPod Touch.
Osmos for the iPhone and iPod Touch features an identical set of content to the iPad release, just for the smaller screen devices. You control an organism that can absorb smaller ones, and you can shoot pieces of yourself to propel yourself about the various arenas and game modes Osmos provides. The game comes with support for the iPhone 4's Retina Display as well, and is available on the App Store right now for $2.99.
The iPad version was well received by iPad owners, making it the number one game on the App Store shortly after release. I too a look at the iPad version and found it to be beautifully designed and a treat to play. This port should hopefully maintain the quality of the original computer versions and the iPad version.