Android (root ): We all know that switching on flight mode is a great way to save battery in low signal areas, but it only works if you notice before your phone dies. Auto Pilot Mode helps by switching it on when you're out of a coverage area.
The app allows you to set a threshold below which your phone will slip into flight mode, periodically checking back in to see if you've got signal again. You can choose how long that mode stays active -- longer periods without checking in saves more battery but, obviously, you miss more messages -- and even choose whether or not it brings back Wi-Fi.
The app requires root for anyone running Android 4.2 and up. If you're on any version of Android between 2.3.3 and 4.1, you can use the app without root. Anything below Android 2.3.3 is not supported.