iPhone and iPad apps tagged "guitar tuners."
Planet Waves Guitar Tools is a Wealth of Guitar Utilities
Planet Waves Guitar Tools for iPhone, iPod Touch, and iPad aims to be the only portable guitar utility kit you’ll ever need. The app has a staggering number of features, including a chord finder, scales, metronome, tuner, and guitar store locator.
Tuner Metronome Minimalist Interface Gets it Done
From the land of sushi and Godzilla comes Tuner Metronome, an app which as the name suggests offers various tuning and tempo tools for everyday musicians. It's developer MIZO claims it to be "the most high quality digital tuner metronome." I wouldn't go so far, but this is certainly a solid application.
Stay in Tune turns your iPhone into a Guitar Tuner
If you play the Guitar, you'll want to get one of the many guitar tuning apps currently available. Of these, "Stay in Tune" seems to be one of the more popular, although Tyro Tuner, Omni Tuner, Cleartune and InTuna all do the same thing -- use the internal microphone to tune a standard 6 string guitar.