123Tuner is a free website to tune guitar. You just play your guitar in front of mic of your computer, and 123Tuner will tell you how off pitch your guitar is.
You do not need to install anything to tune guitar with 123Tuner, and don’t even need to create an online account. Just go to 123Tuner. It will show a “Flash” message box that this application is trying to access your mic. Allow that, as that is how 123Tuner will hear notes from your computer mic.
Once that is done, just play a note with your guitar in front of mic of your computer. 123Tuner will tell you which note is being played. Apart from that, it will also show how off pitch that note it. For this, it uses a very simple to understand interface that shows a dial with a needle. Direction of needle will tell you what type of adjustment you need to make in your guitar.
Once pitch of guitar has been adjusted correctly, needle will become Green.
I am not a musician, so could not test 123Tuner (my music skills are limited to just play piano on computer, and play drums on computer). But it does looks promising, extremely easy to use, and useful. If you are a musician, do share your experience with this in comments.