ELantern

This application is a very simple Lighting application which can use both flash as well as LCD screen light. This also has a auto on/off option which works when you launch the application.So Keep the checkbox ON and when the power goes, just launch the application.It will start the Flash/LCD/Both.
You can also turn OFF this auto option.
Pressing home button, you can do your other stuff while the LCD/Flash is still ON.
No need to again launch the application to turn OFF the Light.There is notification , Just click on that.
Note- If your device is not having flash, Flash will be disabled on app. But even then you can use LCD screen Light source.