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.
This is a library of perfboard and single-sided PCB effect layouts for guitar and bass. I'm not an electrical engineer by any stretch of the imagination, just a DIY'er who likes drawing layouts. It is meant for the hobbyist (so commercial use of any of these layout is not allowed without permission) and as a way to give back to the online DIY community.