Developer: Sonzea, LLC
Price: $3.99 Download on the App Store
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.
I was reasonably happy with "Stay in Tune" -- while not quite as accurate as a cheap Korg or other tuner, its a perfectly acceptable way of tuning an accoustic guitar, and well worth the $3.99.