Dave posted an article that contained a bit about Jury Nullification, but I think it's such an interesting topic that I'm going to add more about it.
For those who don't know (or don't have the time to read through the fairly long article to get to the relevent bits to this discussion), jury nullificiation (JN, for the purposes of satisfying my laziness) is a little-known tool that juries have in a trial. Basically, it means that a jury can acquit because a law is stupid. So, if I'm on trial for painting my house the wrong shade of blue in defiance of a local ordinance, a jury can decide that I'm not guilty because the ordinance is just dumb -- I mean, who gives a rat's ass what shade of blue your house is?
If you think about it, this makes sense -- people always hear a jury's verdict, but no one ever hears the jury's reasoning (unless they choose to divulge it later). A jury can acquit for nearly any reason -- bad testimony, unreliable witnesses, incompetent lawyers, insulting prosecutors... why not because the law is dumb?
The problem, of course, is really twofold: first, the legal system (at least the prosecutorial arm of it) hates it, and second, the entire concept is at odds with conservatives' objections to judicial activism.
Let's start with the first point. It shouldn't surprise anyone that the legal system dislikes JN. After all, it's a direct slap in the face to police and prosecutors. They work hard to arrest and convict, only to have some idiot tell them that the law is wrong, they were wrong, and the defendant goes free? Ouch. Of course, JN protects us from police and prosecutors as well as runaway legislators. Take the recent Martha Stewart debacle, for instance: she was convicted of lying to investigators about something that wasn't a crime to begin with during informal questioning. Exactly the same logic can be applied to me if I tell a cop that the sky is yellow when it is, in fact, blue.
Unfortunately for us, the legal system has the power to do something about this. In addition to juryless trials (quick! Recite the 7th amendment!*), the legal system has rigged the game. Defense lawyers are *not allowed* to tell juries about JN. Defenders who do can face fines and even mistrials. Also, I believe that knowledge of JN can be grounds for removing a potential juror from the jury pool (though I'm not a lawyer, so I can't be 100% sure).
In addition to procedural issues, there is also a philosophical issue to JN that just occurred to me. Consider the Supreme Court: how many times have conservatives complained about "activist judges" who overturn laws because they don't agree with them? The same thing applies here, and with similar numbers. Nine justices vs. 12 jurors -- why is it ok for one group to effectively overturn a law but another can't? One answer I can think of is the fact that SCOTUS is nation-wide, but that's an unsatisfying answer. When you get right down to it, the entire legal system is so precedence-based that there's little chance to get around it -- once a case is decided in one way, it is used to argue that all such cases should be decided that way. And for that matter, this also applies on a local scale -- a county judge can overturn laws just as well as a local jury.
Despite philosophical and procedural roadblocks, I think that JN is a very good thing -- in fact, I consider it a responsibility of the juror to consider the appropriateness of a law in addition to simple guilt and innocence.
Just don't tell a judge that -- you may never get a chance to use it.
*"In Suits at common law, where the value in controversy shall exceed twenty dollars, the right of trial by jury shall be preserved, and no fact tried by a jury, shall be otherwise re-examined in any Court of the United States, than according to the rules of the common law."