HotPaw Guitar Tuner
A strobe tuner is "the most accurate type of tuner" according to Wikipedia. The HotPaw Guitar Tuner provides 6 precision Strobe Tuners for your guitar. Fast. Accurate. Noise resistant. See your notes also appear on a keyboard image and a musical grand staff also, as a great music learning aid. Uses the built-in mic on your Mac, or an external microphone.
With 6 simultaneous strobe tuners, you don't need to wait for your room or recording studio to become quiet enough. You don't need to wait for a tuner to lock onto each note. You don't need to wait for a needle to finish moving. If you're really good, you don't even need to tune one guitar string at a time. With multiple strobe tuners, you can see if one string shifts in frequency while tuning another. You can also see if plucking a string hard on your guitar causes the notes to "bend".
Just watch the waterfall waveforms overlaid on each string. When the waves for a particular guitar string synchronize or line up vertically and stop leaning and shifting left (flat) or right (sharp), you are precisely in tune with an internal equally-tempered digital oscillator.
8 alternate tunings (Standard EADGBE, Drop D, DADGAD, etc.) are supported.
Also available are the HotPaw iChromatic tuner for OS X in the Mac App Store, and the inTuna Strobe Guitar Tuners for iPhone and iPad in the iOS App stores.