We often use the flash on our smartphones as the emergency flashlight. Be it in places like theatre or during a late night party, flashlights are a boon when darkness engulfs us. However, with the lack of uniformity across the UI on Android, using the flashlight which can be triggered on immediately is a real pain. Some of the OEMs allow options of the same under the notifications panel, but still you need to have your device unlocked and ready to go before you can turn on the Flashlight. This could be a nuisance when you want it immediately.
Power Button Flashlight/Torch is a very interesting application available for free from the Google Play Store that basically allows you to map the power button on your Android phone to turn on the flashlight when you need to without the need of unlocking the phone. The best part of the application is that it does not require any sort of rooting or modding your device.
The application by default maps your device in a way that if you press the power button three times continuously, the flashlight will turn on. The light turns back off in 10 minutes, to prevent battery drain, you can also unlock the device and use the giant power toggle to turn the flashlight off. You can also map the power key based on your preference as well as just turn the flash off using the power button too, but you will have to pay $.99 for the same.
You can download the free version of the application from here