Free Online Chromatic Guitar Tuner using Microphone
How to tune your guitar (or other instrument):
If you have an electric guitar – you can connect it via Line-in input. If you have a microphone – you can use it. It is especially well suited for acoustic guitar. You can also bring a laptop (with integrated microphone) to your guitar amp.
Play a single note and watch the tuner.
How to tune your guitar without electronic tuner – you can read here: How to Tune a Guitar Without Tuner