OmniTuner
$5.99 from Mauvila Software 
A versatile chromatic tuner for a variety of instruments. A real, microphone-based guitar tuner and much much more.
FEATURES:
-A sophisticated pitch detection engine that works with most musical instruments, including stringed instruments, woodwinds, and brass
-A fretboard mode that helps you tune stringed instruments, including banjo, mandolin, ukulele, bass guitar, and many more
-A standard note display that accurately reports both the note name and the octave of the note played
-A display that reports the frequency of the note played
-Multiple guitar tunings
-Easy-to-use hands-free interface
-Optional non-standard note frequencies (via changing A4)
-Both LED and analog pitch meters
Compatibility:
iPhone (all types): compatible
iPod Touch - 2nd generation: requires external microphone
iPod Touch - 1st generation: NOT compatible
