If you don't want to go buy a tuner, and you have a computer with a mic, you 
should try this if you need to tune something. As long as you know the frequency
of the note you are tuning to, it will work fine. I was too lazy to write a 
Discrete Fourier Transform that would simply tell the frequency of the note, but 
this is, in my opinion, much cooler. Since the frequency of the note also 
specifies the wavelength of the note, and computer sound cards sample sound at a 
consistent rate (44100 times a second), simply drawing the sound data on the 
screen will allow you to see if a note is in tune, close, or way off, based on 
how it appears to move. With a display of the waveform that is an even multiple 
of the wavelength of the note you are tuning to (both in samples), the waveform
will appear to stand still. If its flat, the waveform will appear to precess to 
the right, and if it is sharp it will appear to recess to the left. If the note 
is way off, it will jump around a lot. Those of you with a scientific and 
musical background will appreciate this I hope. I needed it to tune my new 
viola, so I've tested it and all.
 
~Benjamin Land