8notes has a free online guitar tuner which can be found here:

http://www.8notes.com/guitar_tuner/

Even if you have a clip on tuner it is recommended to go to the 8notes link and listen to the sounds of the six strings as played by the online tuner before you try to tune the guitar yourself. This tuner will highlight in red each string and play the correct sound several times at each push of the play button.