From Electro Harmonix;

The NYC original. Hendrix and Santana were among the
first to get a piece of the Pi, and for over 40 years the Big Muff Pi
has been defining the sound of rock guitar. Revered by contemporary
guitarists and rock legends for its rich, creamy, violin-like sustain,
from Pink Floyd to The White Stripes, everyone still wants a piece of
the Pi!