Soulsonic makes some cool effects. The Hunny Bunny is one of the best. He's released a schematic on the freestompboxes.org forum. First here's the description from his site:
"The Hunny Bunny is what I consider to be my first truly original design overdrive. The basic idea was to get away from the generic “Tube Screamer” style opamp+diodes concept while still having the tone-shaping character that has made that style popular.
The result is a clever hybrid of the opamp and discrete styles: an opamp does the basic tone shaping and drives a MOSFET to create a very natural overdrive.
The timbre of the overdrive is controlled by the “Honey” knob. At minimal settings, the overdrive is tight and crunchy – a very “British” voice that sounds right at home barking out classic AC/DC riffs. As it’s turned up, the overdrive gets thicker and more saturated, and when the Gain control is up high, it’s no longer appropriate to call it an overdrive because it’s become a great heavy sustaining distortion. A simple passive high-cut Tone control lets you tame down the highs if you’re playing through a bright amp."
There's a decent sound clip on his site too. Here's the schematic:
Madbean has produced this PCB layout for the Hunny Bunny: http://www.box.com/s/na1r16nka8vs4fejmgz8
The tone control is just a simple adjustable low pass filter positioned at the end of the circuit.