Subatomic is a game that's available for the iPhone and the iPad. The premise is that you push molecules of different colors around a field with gravity. Portals sit on the gamefield full of molecules, and as they slowly fade away, it's your job to pull more in and match them up. By touching near a molecule, you can push it in a certain direction with a gravity vortex (the game has a cool blurred graphical feature whenever you touch the screen). The game ramps up pretty quickly; eventually you're trying to push molecules toward multiple portals while trying to keep different-colored molecules from reaching the wrong portals.
Unfortunately, the difficulty curve can be a little high. It's sometimes hard to control the incoming molecules, and when you accidentally touch a little too close, you'll find yourself flinging molecules off into the distance instead of getting them where they want to go. At a basic level, though, it's an interesting control scheme that I haven't seen before. The game contains a full tutorial as part of its 31 levels, and the music and backgrounds (some of which contain actual electron microscope imagery from real-life scientists) add to the experience. The social features of the game are managed by OpenFeint.
Subatomic is US$1.99 on the App Store, and a free version (with seven levels to try) is available as well.