HotPaw inTuna A440 is a Free strobe tuner for just one musical pitch, Concert A, commonly 440 Hz, using the microphone on your iPhone for sound input. Many professional musicians prefer strobe tuners because they are faster and more precise, and work better when there is background noise, compared with needle indicator tuners.
The concert A reference pitch used by inTuna A440 is adjustable by plus/minus 10 Hz.
Also by HotPaw Productions in the App store Music category:
inTuna - Guitar Tuner, a strobe tuner optimized for tuning 6 string guitars.
Sing-inTuna, visualize how well you can sing in tune.
and iAutoharp, easy and fun to play.