MXR M103 Blue Box Octave Fuzz
This is the crazy effect heard on Jimmy Page's "Fool in the Rain" solo. The Blue Box Octave Fuzz takes your guitar signal and duplicates it two octaves down for a huge, subterranean tone. The Blend knob lets you control how much of the fuzz tone is mixed with the two octaves down signal. Not for the timid—there is truly nothing like the Blue Box.