Brian of Wampler effects builds some great sounding dirt boxes. This one, as you can probably guess via the name, is supposed to emulate that much loved Marshall Plexi tone. It sounds pretty nice too:
The guys over at freestompboxes.org had a little look at what goes inside one, find the schematic here (and some useful input from Brian himself!);
Vero by Harald Sabro (www.sabrotone.com) –
More build info in the thread: http://www.freestompboxes.org/viewtopic.php?f=7&t=11641