http://en.wikipedia.org/wiki/Jury_nullification
William Penn case covered in the above link.
Most judges will not ALLOW jury nullification to be mentioned in their court even though it is a foundation stone of American history.
That is why we, as Americans, need to educate ourselves about nullification. Also, as much as I believe in the rule of law, our courts no lover serve that purpose. We now have a moral obligation to be "truthful but not helpful" during jury selection, to give the "least untruthful" answers, and to make sure at least one decent person gets on each jury when possible.