A simple formula that will give you the Hz value for any given musical note:

Note in Hz = (440*2^(Note You Want/12))

Note You Want: This is any number you want. 0 is A above middle C. 1 is A sharp. -1 is G sharp. You'll notice the 12 in the equation; this is there because there are 12 semitones in a scale. So, for an A an octave above A above middle C, use 12. I'm sure you can figure out the rest.

This assumes that you are tuned to A440. The 440 in the equation can be changed if you're tuned to something else.

Now you can build a guitar tuner, or make an Impulse Tracker clone.

Sources: My big brother: a bona fide engineer and musician.

NB: Before using this equation for anything practical, please read up on equal temperament, An anomaly in the traditional western twelve-tone musical scale, and Pythagorean comma. (Thanks jkao and Jolter)