MXR 103 - Blue Box.
Takes your guitar signal and duplicates it two octaves down for a huge, subterranean tone.
The blend knob lets you control how much of the effects tone you mix with your dry signal. Not for the timid - this is a crazy effect - and there is nothing else like it.
Hardwire bypass. Powered by 9 volt battery or AC adaptor (RPC921R).