guitar tuner widget?

I don't use widgets much, but there are a lot of small guitar tuner programs that would open quickly. Two that I have used are AWS Guitar Tuner and Freaky Tuner OS X. Freaky Tuner is for multiple instruments.

There's a GuitarChords widget, which shows you how to fret different chords in different tunings. And it plays the chords as you choose them, so this might get you close to what you want. There's no feature just for tuning, however.

Alternatively, you could buy yourself an A440 tuning fork at a music store (they're fairly inexpensive), and tune your 5th fret harmonic on your low E string to that, then tune the rest of the guitar using the 5th and 4th frets (depending on the string you're tuning). This is not an Apple tip, but the way I've been doing it for hundreds of years. 😀

an A440 tuning fork

Actually, that's my preferred method of tuning. Plus, it's always in the case when I need it.

Indeed! I carry mine everywhere!