Jury Nullification is a power that the common people have to get rid of bad laws. Most people do not know of it's existance.
How it works is this, when someone is brought to trial on charges, the jury finds him Not Guilty inspite of overwhelming evidence of his violation. Once the person is found not guilty (usually by one vote), he can no longer be tried for the crime.
The benefit of this is that the finding of not guilty can be used in future trials as proof that the law is bad and it will eventually go away.
One thing to note is that many judges do not want juries to know of thier power and will remove jurors whom they suspect may excercise their nullification power.
HERE'S A GOOD WRITE-UP ON JURY NULLIFICATION.