one of my favorite apps for the ipod touch lately has been the AmbiScience series from Tesla Software. these apps play loops of ambient music which can be combined with various brainwave entrainment sounds, including settings for increased alertness, meditation, and deep sleep (i definitely recommend reading up on brainwave entrainment if youre not familiar with it already, its really fascinating). the apps work great as a sleep aid, and a timer is included which will shut down the app after a certain time. even without the entrainment options, the ambient loops are very enjoyable to listen to, well-produced and usually long enough so that the loop is not immediately noticeable. the Pure Sleep and Brain Power editions feature more organic sounds somewhat in the style of Steve Roach or Michael Stearns, and Androids Odyssey naturally has a more electronic, classic sequencer spacemusic feel.
and as an added bonus, An Android's Odyssey is free at the iTunes store for the next few days. definitely recommended for fans of ambient music who own an iPhone or iPod touch!