A free program for iPhone, by Karl Morton.

PitchLab is the world’s best hands-free, portable tuner for guitar, ukulele, banjo, violin, mandolin, and other stringed musical instruments. The app is perfect for beginners, professionals, and music educators. It is designed to be used by musicians and music students who want to tune their instruments without the hassle of using a traditional tuner.

PitchLab helps you tune your guitar with the convenience of your smartphone.  You can tune to the standard tuning for your instrument or to a custom tuning that you have created. You can use the app to tune your guitar to any string or note.