The ENO tuner mentioned above is quite useful if you want to tune thigs which aren't a guitar.
It has a VERY wide set of jaws for clipping to something thicker than a guitar headstock. Mrs Duck has one and it clips to the body of an autoharp with no trouble. My Intelli will not reach, and I had to clip that to a tuning pin, which was not a very convenient solution.
The Eno also has a built in microphone which can be selected for checking tuning on a wind instrument such as a whistle. It will even work on a voice if you want to test your ability to hold a single note.
Quack!
GtD.