I don't need such a thing for sleeping, but I find SimplyNoise gives a satisfactory sound having a much steeper fall-off with frequency than white noise (flat spectrum of energy vs. frequency) or pink noise (3dB fall-off per octave), both of which sound unpleasantly harsh to me. They also have a few soundscapes (thunderstorm, river, etc.). The app is not free, but cheap, and there are also pay-what-you-want mp3 download files.