If you're looking for the most accurate instrument and calibration tuner for iOS, then you've found it! UltraTuner is the most precise digital tuner available for iOS, providing precision down to .01 (yes, that's 1/100th) of a cent. UltraTuner is 10x more precise than mechanical strobe tuners, which are generally considered the "gold standard" of precision tuners, and they typically don't fit into your shirt pocket. UltraTuner is a standalone app for your iPhone and iPod touch and is with you everywhere you go.
http://www.ikmultimedia.com/ultratuner for more information.