Here's a very cool synthy sounding octave down / up invented by Slacker (the creator of The Echobase). Original thread here. I included the Octave up addon from the schematic aswell. I had to use shitloads of cuts and links in this layout, but at least I got it into a good size. :)

I did have problems getting this one to work/track properly. Only the low E and A strings tracked and I had to hit the strings hard. Here is what I did... Probably not the best solution lol, but for now it's good enough until I have more time to try out other things. If you want to build it 1:1 with the schematic, just omit the 47K resistor. Here's a short soundclip of the circuit/layout (with my fix):

I'm using humbuckers, neck pickup, mix at 50%, Octave up maxed and 2nd octave down selected.

Another Octave Down, very different from the Synthbox. Cleaner and bassier octave.
Modded by me for true bypass. Have a look at my completed build here.

This circuit is a bit below unity gain, but John Kallas did his own version of my layout with an added boost daughterboard. Check out his layout here.

Here's a layout for a very nice DIY octave down by Liquids (forum member @ diystompboxes) based on Joe Davisson's Shocktave. It's a very cool "synthy" sounding octavde down that tracks fairly well.

Here's a layout for the original Shocktave (not verified!)