Boss OC-2 Octave
The OC-2 doesn't fill the same role as, say, my Blue Box-- it doesn't produce a thick, standalone lower octave that can be played by itself. This pedal doubles the input signal an octave down, thickening up the tone of single note runs. Like many octave dividers, it's not very comfortable with super low notes (and isn't useful down there, anyway), but for higher playing, there's a great deal of character and depth to be mined here.
This particular octaver is particularly usable because it offers level controls for the dry signal, octave one, and octave two. This allows for both a single octave blend equal to the original signal, a clean boost with subtle layering of octaves, overwhelming octaves with a little clean for note definition... etcetera, etcetera. There were never many (if any) octave boxes with this kind of flexibility and sound quality, and now that this one is out of print and the digital revolution has made a few of us look like vinyl collectors in an MP3 world, we're probably not going to see these things again.