Here's another design by Martin from Soul Sonic FX. Here's what he had to say about it over on FreeStompboxes:
It's a cool little hybrid combo of an opamp driving a MOSFET. It has a great range of drive, from light and crunchy to thick and heavy. At it's highest settings, some people might consider it a distortion, I guess it just depends on how hot your pickups are. You can see some fun tricks used in this circuit; subtle things that give it the special charm it has.