This one's been making the rounds (Heh!), but I'm sharing it anyway, because it's insane and mind-blowing and I want everyone to see it. (Oh, and here's a video explanation from the folks at BrainStuff - How Stuff Works.)
The helicopter seems to be hovering magically.
Attribution(s): Video, images, and stills are the property of Chris Fay, who now has a specific "Helicopter Video" page on his website in order to deal with its viral notoriety; "Helicopter" by Ian Fuller via Visual hunt (CC BY-NC).