iPhone 4/Android: LED flashlights 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/Android app LED Light turns your phone's LED into a flashlight.
Available for the iPhone 4 (the only iDevice that currently has a flash built in) and Android, 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 want to put that light to good use when you're running around in the dark.
LED Light is a free download from the iTunes App Store or the Android Market (search for LED Light in the Market). 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.
Update: Several commenters appear to be misinterpreting this post, so some clarification: No one's claiming that the iPhone is the only phone with this capability. But it's there, and if you've got an iPhone 4, it's a useful app for keeping that light switched on so you can use it as a flashlight. I've also updated the post to add an Android version for Android phones with LEDs.
LED Light [iTunes App Store]