I play the guitar a little bit and so I was glad to discover this program. The problem with my manual electronic tuner is that it's always running out of battery power so half the time I'm playing completely out of tune. However, Free Guitar Tuner totally eliminates this problem by running directly through my PC.
The principle is very simple. You've got six strings and to pluck each one you just click the mouse on them. There are three main settings - acoustic, electric and synthesized guitar. Just keep clicking the string until the tone on the program sounds exactly like the tone on your guitar. The disadvantage however is that, if you're tone deaf like me, it's hard to ascertain when you've got it right. If only the program could "hear" the guitar through the microphone, and show you how close it was to being in tune like a normal tuner, it would score much more highly.
Nevertheless, for a simple and elegant looking tuner, you need look no further.