A phaser effects pedal made by the Electro-Harmonix company. Widely used by rock and electronic music artists since its introduction in the 1970s. Sounds people claim to be able to emulate using this pedal include those of Korn, the Smashing Pumpkins, Radiohead, Jean-Michel Jarre, and so forth.
The Small Stone is extremely simple to operate: one knob to control the LFO rate, and one switch for "color", plus the stomp switch. As of this writeup the pedal can be had in a Russian-made version from Sovtek, or a new American-made reissue model; of course, there are still a great many used ones floating around.