Jury nullification was intended by the Founders as a powerful weapon against a tyranical government. Even many Americans today believe that *law* is whatever the government SAYS it is......nothing could be further from the truth.
In this fashion William Penn (who gave his name to Pennsylvania) was acquitted by a jury in London in 1670 when prosecuted for preaching Quakerism. The Salem witch trials ended in 1693 after 52 consecutive jury acquittals. Similarly, John Peter Zenger was acquitted by a jury in 1735 in New York when prosecuted for criticizing the mayor. The Fugitive Slave Acts were effectively nullified by juries in the 1850s. Similarly, juries effectively put an end to Prohibition during the 1920s.
Here's an informative web page on JURY NULLIFICATION
My opinion is, if as a potential juror you think a law is unjust and you can't enforce it in good conscience, you should recuse yourself from the jury.