Install Gibson Learn & Master Guitar on your iPhone and you will be able to tune your guitar, as well as other instruments with ease - iPod Touch users can do the same providing a microphone is attached. There is support for a number of different alternate tuning, so if a song you want to play requires drop D tuning but you are unable to tune by ear, this handy app is available to help you out.
Anyone looking to learn to tune by ear can switch the tuner into standard mode where you can tune each string to a sample tone - you can always switch modes again to see how you fared.
If you're just starting out on guitar, or even if you have been playing for some time, the chord library is incredibly useful. This database of essential chord shows you where to place your fingers to play a wide range of open and barre chords.
The app also includes a metronome to help you to keep time whilst playing and there are a number of video tutorials available to help you learn new techniques.
This is not the most accurate or comprehensive app for guitar players, but it is free and worth checking out.