Here is something pretty cool for the dexterous axe slinger that doesn’t have enough room on his/her board for a conventional wah pedal. Effects builder Chris Mahoney has just release his Wahsp mini-wah stomp pedal. The guts of the Wahsp wah is based on the famous Clyde McCoy pedal, and the results are truly outstanding. Here’s the trick though. There is a large, side-mounted knob on the Wahsp that acts as the treadle. When you roll your foot across it back and forth, it makes the traditional wah sound.
If someone were clever s/he would do the same thing with expression/volume pedals and other stompboxes.
(from what's that dude play blog)