The retro stylings of Bubble Sort by Heavy Six Entertainment do well for the nostalgic gamer. It's classic 80's game play in the palm of your hand, and while the graphics and sound effects are something straight out of the Tron era, its got all the benefits of the iPhone platform. It's the authenticity of the design in Bubble Sort combined with a frantic race against the clock that makes it great.
A free version of Bubble Sort is available that has six levels. These levels are relatively easy. Where the game really starts to challenge you is in levels 7-20, which are available in the paid version for 99 cents. The idea is that you have three objects you have to sort into their respective areas as soon as they hit a sorting square using that area's button. It's stupidly simple to learn, but the challenge comes when the objects move faster and faster and become more difficult to tell apart.