Generative, ambient music by Brian Eno. If I need to say more, it's also a mini-sequencer: Drop your finger on the liquidy pastel screen, play a note, make simple loops. Music For Airports that you can make yourself. In an airport, even.
A lot of people buy Nintendo DSes for the sole purpose of having those Dr. Kawashima-approved brain-training games at hand. With this app, you get the exact same mind-juicing benefits for a few bucks, on hardware you already carry.