A couple days ago, Rob Miller was lucky enough to see and photograph a dark Northern Harrier. While dark forms have been documented for various species of harriers, this is only the 2nd dark Northern Harrier ever photographed and only the fourth report of one (as far as I know).
http://www.nemesisbird.com/2012/01/the-dark-ghost/
http://www.nemesisbird.com/2012/01/the-dark-ghost/