Harmonics on Wikipedia

http://en.wikipedia.org/wiki/Harmonics#Harmonics_on_stringed_instruments

A good article on harmonics

 

http://en.wikipedia.org/wiki/Guitar_harmonics#Harmonics

Harmonics specific for guitar

 

http://en.wikipedia.org/wiki/Moodswinger

An article on the Moodswinger, a really weird instrument used by The Liars that relies on overtones