Bugatti Veyron
I saw this on Top Gear last week: a Bugatti Veyron. It blows all the other supercars out of the water. It is the fastest, most powerful street legal car on the market, and it can leave a formula 1 car in its dust.
It has a 8 litre, 16 cylinder engine in a W arrangement (like two V8s merged together) that puts out 1001 horsepower. It has 4 turbochargers, 64 cylinders with electronic continuously variable cam timing, and has something like 10 separate radiators to deal with all the heat.
All this gives it a top speed of over 400 kilometres an hour.
With that kind of speed, the biggest problem is keeping it on the ground. (The takeoff speed of a 747 is only 250km/h). It has a hydraulic rear spoiler to help keep it down, and one of the radiators is solely for cooling the hydraulic fluid of the spoiler. The side mirrors too help generate downforce to keep the car on the road.
At top speed, it uses 5 litres of fuel per minute, and it's fuel tank is big enough that it can go for about 12 minutes at top speed. If you can afford the $2.3 million to buy it though, the fuel consumption is probably not a concern.
You can check out its specs at wikipedia or let Jeremy Clarkson explain why this car is the 8th wonder of the world.