Really nice and as impotent easy to use guitar tuner.

Get it here.

Features

• Tune to absolutely any note you want.
• None of the constraints of conventional tuning patterns.
• Highly accurate pitch detection.
• Simple and intuitive interface.
• Tune your guitar through any available sound input device including Microphone or Line In.
• Excellent tool for checking fret board intonation.

Checkout APtuner as well.