Another version of ad hominem that has been seen around here: anyone who makes a statement I disagree with is only doing it for the money. Anyone who has ever been paid for writing a book, making a movie, making a speech, singing a song, serving in a political office -- can be discredited simply by pointing out that they were paid, or are hoping to be paid. Except the people I agree with, of course--they're doing it out of the highest principle.