gTuner has many features macintosh users and guitar players really need: first of all it helps you to tune your guitar - either in auto-mode, in which gTuner finds the matching string or in the manual-mode, where you can select the current string and tune guitars that are badly out of tune.
With Dock Tuning enabled, you can see if your guitar is tuned up even while you work with other applications. And this is possible because gTuner does not use much CPU power, too.