This is the two best guitar tuners online that I could find, and have been using them for years. Having your guitar strings in tune is a critical part of getting better at guitar. If you are not in tune, you will not sound right, and will be less motivated to move on with your learning. Just a reminder, learning to play guitar should be fun, not something you hate to do, so please learn how to tune your guitar correctly – so that you can be in the correct key.
I have also put up a video of how to tune a guitar for the true beginners out there.
Guitar Tuner courtesy of the folks at Wimpy Player
