BBE OptoStomp is a good one but if you can find a used Carl Martin Classic Opto I'd go with that.
I've been interested in the Classic Opto but the size is a real turn off. If anyone knows of a gut shot please direct me there because I've thought about trying one and re-housing if I like it, but I want to know how big the board is.
Here's a picture to put the size of this thing in perspective.
I don't remember the board size too well, but I do remember it's nowhere near as big as the chassis. Along the same lines, and in a smaller package, keep the BYOC Optical in mind.