The Cardinal Harmonic Tremolo is an optical implementation of the tremolo/vibrato effect found on certain Fender amps from about 1960-1964. It's a rare effect in a stompbox, an unusual, and beautiful, combination of mild phasing and amplitude modulation. This version has a waveform control and a switch to go between harmonic and normal tremolo modes, and a "bright" mode in the middle that sounds more like a vibe.
If you're interested in building one of your own, I have posted schematics and layouts on my website and in the forums at DIYStomboxes, Madbean, and Build Your Own Clone. There will also be a PCB project for this released in the coming months.
This circuit was adapted from or based partly on RG Keen's original FET-based implementation and CultureJam's Shoot the Moon tremolo (which was in turn based on the Tremulus Lune). Thanks to Jimi Photon for the inspiration and Samhay (and others) on DIYStompboxes for helpful suggestions.