No need for an app it is built in
1. Double click the home button to bring up the multitasking bar.
2. Swipe right until the last set of icons is displayed in the multitasking bar (these will include play/pause and skip buttons for iPod or Pandora).
3. The furthest left icon looks like a circular arrow - touch this icon to enable or disable rotation lock on the iPhone.
4. A rotation lock icon will appear in the status bar next to the battery charge icon when rotation lock is enabled. This looks like a circular arrow around a padlock.
When rotation lock is enabled apps will not respond to the accelerometer sensor.
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