I've always loved the sound of thunderstorms, and now I have some great ones on my iPhone. A company called Naturespace has recorded 65 thunderstorms from around the U.S. and put them into the Thundergod app. They use stereo effectively to track the thunder across the sky so that you feel like you're actually in a thunderstorm. There's a sleep timer so that you can fall asleep to these relaxing sounds. The app isn't cheap, at $9.99, but the quality of recording is excellent.