I have been using an iPhone app called Ambiance for a long time, it even allows you to record your own sounds, works in the background, has a sleep and alarm modes etc.
The only down side is it tries to do too much, they have like a sound store and stuff, which is a cool feature but affects the simplicity.
The only down side is it tries to do too much, they have like a sound store and stuff, which is a cool feature but affects the simplicity.
http://ambianceapp.com/