Hard-guitar.com, Get-tuned.com, and Proguitartuner.com offer free tuners online without download. To use these tuners online, you must install the latest version of Adobe Flash, or a similar program. Proguitartuner.com offers apps for your iPhone or Android device.
The tuners referenced are set to the standard EADGBE six-string tuning. Get-tuned.com allows you to choose from other preset tunings, or you can adjust the notes manually to tune by ear. Proguitartuner.com and Hard-guitar.com require you to either plug your guitar into the computer or play it through a microphone in order to read the note's pitch so you do not have to tune the guitar by ear.