A free program for Android, by Brink Technologies.

The Power Button FlashLight app for Android phones lets you activate the flash on your phone by pressing the power button three times in quick succession. With it, you do not have to unlock your device just to turn on the flashlight!

Aside from that, it also allows activation by shaking the phone when the screen is lit.

This app works with all phones that have a power button and an LED, even those that don’t have a camera. Moreover, the app is free to download and use, and it has no in-app purchases or advertising.