In the old days (real old days) juries could vote not only on the guilt or innocence of the defendant, but also on the validity of whatever law was broken in the first place. Basically, it's a common citizen's right to veto a law by refusing to convict. (Of course, everybody thinks they like this idea until they realize some of their pet laws could face the same scrutiny).
This site explains it much better: Fully Informed Jury Association