Here are a couple of online and downloadable guitar tuners. These utilities let you hear the notes you need to tune your guitar in standard tuning, and for some, in alternate guitar tunings, such as DADGAD.
Free Online Guitar Tuners
Here are some sites that offer online guitar tuners. Play the reference notes for tuning your guitar by ear.
Online Tuner at Guitar For Beginners for tuning your guitar in standard (EADGBE) tuning.
Online Tuner at Get Tuned features a chromatic tuner for tuning your guitar in standard tuning and in alternate tunings, such as DADGAD.
Download Guitar Tuners for Free
You can download guitar tuners for free. Here are a couple downloadable guitar at your computer when you’re not connected to the Internet.