Tawch is an easy to use, lightweight app, that turns your phone into a useable torch/flashlight.
Either by using the built-in camera flash, or using your phones screen.
When using your phone screen you can drag your finger up and down the screen to change the brightness.
This is app is still in beta, so would appreciate your feedback on devices it works on/doesn't work on.
The app is now open source: https://github.com/jshtylr/Tawch