Another guitar-tuning trick: if you don't have a reference tone at hand
(and if you don't have absolute hearing), but you do have a television,
you can use the screen-frequency to tune your guitar.
The television screen flickers at 50 hz (at least, in PAL countries).
Look at your g-string with the tv screen as background (holding your
guitar sideways), and tune it until the you see a static waveform
(it's much like looking at an oscilloscope). A g is at 196 hz, so you
will be 4 hz off.
With NTSC sets, I guess you could use the b which is at 123 hz, 3 hz
above the 120 Hz flicker.
maarten