The Flash Tuner app lets you tune your guitar visually using the flash of your iphone. Find a dark room, select the string which you want to tune and point the flash of your phone to the guitar. Once the selected string is in tune, it appers like it stops in the air once you pick it. It's due to the fact that the string and the flash's strobe frequency are the same. Don't expect superior results, as the flash frequency is not 100% precise due to background processes on the phone and other limiations. It's more about having some fun and experimenting with this kind of tuning.
If you have any questions or suggestions about the app, do not hesitate to contact me at foldenyi DOT tamas AT icloud DOT com.