You have a right to trial by jury in this country. The Constitution says "The Trial of all Crimes, except in Cases of Impeachment, shall be by Jury;". The government routinely gets around this by defining most things they don't want you to do as "not crimes", also known as infractions, misdemeanors, etc. But what if the government passes a law that most people think is unjust? What recourse do you have as a citizen other than constantly contacting your representative or filing an expensive lawsuit?
Jury nullification occurs when a defendant is clearly guilty but a jury refuses to convict based on a moral opposition to the law in question.
The most famous example is the Fugitive Slave Act of 1850. Escaped slaves were required to be returned to their masters, even if they were captured in the northern states where slavery was illegal. Anyone helping them to escape and remain free could be charged under the FSA. However, if someone was brought before you as a juror, charged with helping a slave escape, you could vote to acquit based on the immorality of the law and the institution being defended.
Jury nullification is the last defense before armed rebellion against unjust laws. The government can pass a law, the courts can uphold the law, but if the people refuse to implement the law then it fails.
So why haven't you heard of this? Well, there are some issues. First of all, judges don't like it when you usurp their power as arbiters of all that is right in the courtroom. Prosecutors don't like it when they build an airtight case and you overturn all their hard work. The government generally doesn't like it when you don't follow the laws they passed for your own good. There is also some evidence that defendants get treated differently depending on how they appear to the jury, a sympathetic defendant accused of the same crime as a non-sympathetic defendant is more likely to have their conviction nullified. This last is an issue but in my issue doesn't eliminate the need for jury nullification as a check on overbearing government.
Jury nullification can also be used as a tool to overturn unjust law. In 1992 California passed a law requiring that all motorcyclists wear helmets. You can argue about whether motorcyclists should wear helmets, but in a free society I hope we can agree that the government shouldn't be making that decision. The law was an infraction as mentioned above, so you don't get a trial by jury. A judge (actually a traffic commissioner) rules on your guilt or innocence if you choose to contest. Generally they have no incentive to actually rule on the facts of the case but rubber stamp most tickets unless there is an egregious violation by the officer. California has a twist in their law, however, that requires repeated infractions of the same law to be upgraded to a misdemeanor, presumably to punish repeat offenders more severely, and certain types of misdemeanor get you a trial by jury.
So imagine this situation. You are standing in front of a judge and jury. You have been accused of not wearing a proper motorcycle helmet. The jury, if they convict you, will be sending you to jail. For not wearing a motorcycle helmet. If you could tell them about jury nullification how many people do you know that would send someone to jail for not wearing a motorcycle helmet when they rode? Many states, however, make it illegal to even mention jury nullification. You can be held in contempt, your trial can be declared a mistrial, and as a juror you can be prosecuted if you don't disclose the fact in advance that you think jury nullification is a legitimate tool of the citizenry.
The Fully Informed Jury Association has been working for years to try to ensure that jurors know their rights. You can see what they are doing and read their arguments at their website http://fija.org/
Also, here is a short (but entertaining) video on the subject.
Enjoy, but remember, if you think this makes sense you will probably never get to serve on a jury. You threaten their power.