Justin Everman makes a custom octave pedal, I forget his website..but its a high end sweeet sounding pedal. I know him personally and he does all this by hand and he likes getting the Military grade parts....just cool stuff. Funny he sells pretty big in Japan for some reason, a side home business engineer.
It adds a note one or two octaves above and/or below the note being played. The solo from Purple Haze, the intro to Jet Airliner, and the solo to Fool in the Rain are all examples you already know.