Over a year ago on Boing Boing, I linked to this video from a guy who made a propeller-powered vehicle that he claimed could travel downwind faster than the wind. Some people think it was a hoax, and some don't.
In Make Vol. 11, Charles Platt made a miniature model of the vehicle and came to the conclusion that there is no such thing as a wind-powered vehicle that can travel faster than the speed of the wind.
Now there's a new video on YouTube (above) that claims it is possible to sail directly downwind faster than the wind (aka DDFTTW). You can read heated discussions about the video and its claims at Makezine, the Mythbusters Fan Club discussion board, and Randi.org. The creator of the video, spork33, hopes that the Mythbusters folks will attempt to replicate the experiment.
I admit that I don't understand the physics involved, so I don't really know whether DDFTTW is possible, but I am siding with Charles on this because I've never known him to be wrong when it comes to math, physics, or electricity.
UPDATE: Charles says: "You might make it clear to readers that the argument is strictly confined to the behavior of vehicles in a direct tail wind. A cross wind can indeed create a force-multiplying effect when it blows against an angled sail."