The MXR Blue Box became the go-to octave effect and a gear icon after Jimmy Page used it for his solo in "Fool In The Rain." It's incredibly simple - two knobs for Output and Blend - one of those pedals that just does its thing and sounds good without a ton of dialing in. It tracks your notes and doubles with a two-octave down copy, providing a bassy, raw feel. Put it front of a fuzz and stand back.
This little box is lightly used and functions perfectly. It ships the next day.