An easy to use guitar tuner iPhone app. It's built using sophisticated signal processing to get just the right balance between precision and responsiveness. Thanks to some even more sophisticated signal processing tricks it also does a good job in noisy environments.
You might not be able to retune your guitar during a concert, but you won't need to shout at your friends "Stop talking, I'm trying to tune my guitar!"... or least not as often as with some other tuners.
This is all well and good, but the reason you'll stick with this tuner once you've tried it: It looks really cool!