This is a title which one Matthew Hopkins awarded himself in 1644, in the town of Manningtree, Essex, UK. Hopkins made a good living travelling around the countryside with his assistants, charging a flat fee plus expenses, plus a bonus for each witch exposed and executed.
After a while, Hopkins wore out his welcome, and people began to think that he "found out witches not by God's aid, but by the Devil's". He was tried for witchcraft by his own rules (which pretty much guaranteed a conviction) and, hoist by his own petard, came to a quick end.
Nowadays we call these people "consultants", but for some reason the congenial custom of tying them up and throwing them in ponds has been lost.
Facts courtesy of Extraordinary Popular Delusions and the Madness of Crowds by Charles MacKay, 1841.
(Update, April 2011: I originally said this all happened during the reign of James I, who as it turns out had gone to his dubious reward in 1625, well before 1644. I find that I have absolutely no idea whether MacKay really said that or not, and little will to find out. Thanks to Catchpole for rattling my cage about the error.)