The Big Muff Pi is Electro-Harmonix’s classic fuzz/sustain pedal, first released in 1969. It’s known for its thick, violin-like sustain and heavy distortion — less of a sharp “fuzz” and more of a wall-of-sound saturation.
Famous users: David Gilmour (Pink Floyd), Jack White, Billy Corgan (Smashing Pumpkins — “Cherub Rock” and most of Siamese Dream), J Mascis (Dinosaur Jr.), and Kevin Shields (My Bloody Valentine, in modified form).