Invented in 1752 by George Frideric Handel's trumpeter John Shore, tuning forks are simple metal two-pronged forks which resonate with a pure tone when struck. The tone that a particular tuning fork generates depends upon the length of the two prongs, and cannot be changed on the fly; one tuning fork generates one tone.
A tiny quartz tuning fork is used in quartz digital watches. The peizoelectric properties of quartz crystals causes a quartz tuning fork to generate a pulsed electrical current as it resonates, which is used by the computer chip in the watch to keep track of the passage of time. (anyone know what frequencies digital watches usually use?)