Jew's harp

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A small musical instrument consisting of a lyre-shaped metal frame that is held between the teeth and a projecting steel tongue that is plucked to produce a soft twanging sound.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A musical instrument consisting of a flexible metal or bamboo “tongue” attached to a frame. This tongue is placed in the performer’s mouth and plucked with the finger to produce a note of constant pitch. Melodies can be created by changing the shape of the mouth and causing different overtones.