Conventional wisdom says that a wind-powered vehicle without stored energy, when moving downwind, can only move as fast as the wind itself. Though some racing boats and yachts can move faster than the wind, they're actually sailing into the wind at an angle, thus being driven by the apparent wind, not the actual wind.
When going directly downwind, it would seem to be impossible to move faster than the wind. As you approach the wind's speed, the wind's power lessens, and by the time you reach the wind's precise speed, you'll be receiving no help at all.
Rick Cavallero had a different idea, and with the help of the San Jose State University aero department, Google, and Joby Energy, he built this windmill-type car made mostly of foam.
The idea is that the wheels and the propeller are attached, but in the opposite way you'd expect--the wheels drive the propeller, not the other way around. So the wind pushes the car forward, which turns the wheels, which turn the propeller, which pushes the car forward, which turns the wheels, and so on.
That sounds suspiciously like the impossible perpetual motion machine, but in my understanding of the vehicle, it's not. The car is able to accelerate past the wind's speed thanks to the propeller (being turned by the wheels), and since the car still has some speed, it'll keep accelerating for a little while. But since the wind's power on the car has now dropped to zero, the car will eventually succumb to friction and slow down until it moves slower than the wind's speed, at which point the whole process will start up again.
That's my understanding, and from the comments this vehicle has attracted on other blogs, everyone seems to feel differently. The car may turn out to have little value to the auto or alternative energy industries, and remain an intellectual curiosity--but then again, it may not.