Next up. Not quite as obscure.
The Theremin:
Invented by Léon Theremin in 1919. The Theremin truly is a freak among instruments, it's often associated with the 1950s and science fiction movies. It is unique in that it is the first instrument to be played with out touching. Its made up of two radio frequency oscillators and two metal antennae, which are very sensitive to even the most subtle vibrations and movement. One of the antennae (the one that looks like a loop) controls volume, the farther your hand is from it the louder it gets, and vice versa. The other one controls pitch and is the one used to play notes. Players offen shake their hands to get a clean vibrato sound, which is what Theremin more than anything else is known for, it's tone and effect is very similar to that of the musical saw.
The instrument never became a real commercial success. It did however gather a large cult following and it's influence and impact on modern electronic music can still be heard today. Robert Moog was a theremin enthusiast and he studied the instrument before creating his own masterpiece, the Moog synthesizer.
Rock bands who have used or experimented with the Theremin include Jimmy Page, Pere Ubu, The Decemberists, Patrick Wolf and Fishbone among others.
A demonstration.
YouTube - Theremin Lesson One
A similar instrument created in 1928 is
The Ondes Martenot:
The fundamental difference between the Ondes Martenot and the Theremin is that the Martenot needs to be touched, it consists of a keyboard and a sensitive string tied to a finger ring and a strip control to manage virbato and glissando. It also has built in pitch and volume control, filters and loud speakers.
Popular musicians who have used the Martenot include Jonny Greenwood of Radiohead and Frank Zappa.
Demonstration.
YouTube - Jean Laurendeau and the Ondes Martenot