iPhone 4 only: LED torches are excellent, low energy devices that can put out serious light. Several phones — including the iPhone 4 — use LED lights as camera flashes. Free iPhone app LED Light turns your iPhone's LED into a torch or strobe.
Available only for the iPhone 4 (the only iDevice that currently has a flash built in), LED Light is a pretty simple, single-use app. It turns on your light as soon as you launch the app, and it turns off just as easily. There's nothing more to it beyond that, but it's definitely worth downloading if you've shelled out for an iPhone 4 (or, in the Australian context, plan to do so).
LED Light is a free download from the iTunes App Store. If for some reason you don't like LED Light, you can also try FlashLight4G, the other LED light for iPhone 4, and the app pictured in the screenshot above.
LED Light [iTunes App Store]