Thunderspace is an audio thunderstorm simulator with 3D stereoscopic sound that's perfect for relaxing with headphones, and it has one especially cool feature.
Unlike other audio storm generators, Thunderspace adds synchronized simulated lightning. Even further, the 3D stereoscopic sound makes the storm seem as if it's all around you. You're not going to get this experience simply by playing the audio through your iPhone, so you should plug in some earphones. I tried with both earbuds and headphones, and the latter definitely brought out more of the bass, which is especially cool when there's a lot of thunder.
Thunderbolts and lightning
The feature that sets Thunderspace apart from other apps is the capability to simulate lightning. It uses the flash on your iPhone to send one or more light pulses and soon after you'll hear the thunder that matches the lightning strikes. The idea is that you'll start a "storm," then place your iPhone face-down on something so it's out of your direct view. This way the lightning seems more realistic than actually seeing the flash on your iPhone go off. It's not perfect, and I noticed some lightning flashes that weren't followed by thunder, but most of the time it's quite realistic. Don't worry about the screen staying on either; Thunderspace uses your front-facing camera to detect when your iPhone is face-down and turns off the screen immediately.
The two storm types that come with the app are Roof Garden, giving you strong winds and heavy rain like in a summer storm, and Waterscape, a lighter rain falling into water, with thunder from farther away. There are six additional storm types and you can demo each one, but you'll have to pay 99 cents each to add new storms to your library.