CentTuner is a responsive and accurate chromatic tuner. It uses the iPhone's microphone for determining the pitch. The detected pitch is given straight away, as the closest note, and a meter tells you if the tone is in tune, or it is too flat or sharp.
The Best Features
Responsive and optimized - several results each second.
Accurate within cents of the tone.
Simple and clean user interface makes it easy to use.
All this for free.
Background is a modified version of a photo by Feliciano Guimarães from Guimarães, Portugal (Electric guitar Uploaded by tm) [CC BY 2.0], via Wikimedia Commons