Jury Nullification
I just learned today about an apparently little-known feature of English common-law (and hence part of the legal systems of the UK, Canada, Australia, New Zealand and the US)): juries may ignore the law and the facts and judge merely by their conscience.
Stallman.org passed on a good article on this (although it's US-centric), but the salient point is that juries are no longer explicitly told of this right (indeed, Duty, if you're Thomas Jefferson).
So, if you're ever in a jury, remember: it is your right to rule against the law if you think the law is a stupid one (but don't tell the prosecution lawyer about it before the trial). Also, there are apparently court rules against defence lawyers telling (or "admonishing") the jury to use this right (or even telling them it exists).