The app measures sound pitch using your phone's microphone, letting you tune any musical instrument (violin, viola, violoncello, bass, guitar, ukulele, piano, even wind instruments to name a few that have been tested).

Features include:
1. orchestra tuning (shifting/redefining tone frequencies, e.g. A440 > A443),
2. different temperaments (pythagorean, bagpipe, etc.),
3. custom user-defined temperaments,
4. different note namings (English, Latin, Hindustani),
4. a nonlinear scale,
5. microphone sensitivity setup,
6. pitch pipe.

 
https://play.google.com/store/apps/details?id=org.cohortor.gstringsng