As discussed in the 9/11 post, I firmly beleive the EU will be the next super power.
Not any time soon though. At present it would take all of Europe + Japan (Japan's cool, it should be European) to match US output. However, no Empire lasts forever. Its only a matter of time before the US's crown falls and someone is their to pick it up. And with any luck, it'll be the Europeans..... or also likely, an industrialised China .
"Wise men talk because they have something to say; fools, because they have to say something." -Plato