One of the essential apps on my myTouch has become Relax and Sleep, a program that allows you to mix and match various soothing sounds and music in an ad hoc way. Its interface is both simple and powerful (as the screenshot below eludes to). Most importantly, it'll play your selections without interruption pretty much forever (or until your battery dies). Note: you'll want to make sure to add this program to your task manager's ignore list - otherwise you'll be stumped as to why the music only plays for 5 minutes and then stops.
While I think this application would be invaluable for catching Z's on an airplane or otherwise sleep-hostile environment, what I've been using it so far is to create a sleep-lulling state for our 1 year old. Take last Sunday, for example. We wanted to put him down for a nap at a stranger's house. My theory is that the music (relaxing rhythm + crickets was the pick of the day) allowed him to drift off to sleep a little faster, rather than being distracted by the new surroundings.
That's probably a load of crap. He probably slept no differently with or without the music - but it made *me* feel better. It made me feel like he was in a chill atmosphere, rather than a strange room with strange apartment sounds.
If you travel, work in a stressed environment, have trouble sleeping, desperately miss the sound of a running creek outside your window or have a baby, you should give this app a try.
This is a free app and without any annoying limitations that force you to upgrade. You'll upgrade this app because you like it so much, which is probably how it should be anyway.