The US will continue to be very powerful, but won't be the sole superpower anymore. I agree with Marshal Zukov, the situation will likely look somewhat similar to how the world looked in the 17th or 18th centuries - several large, powerful countries or empires, none quite powerful enough to destroy the others entirely, balancing each other out.
As for who these nations will, the US and China are the favorites, and the most obvious. Barring something drastic happening (A complete collapse in the central government, or a nuclear sneak attack on either - unlikely) that prediction will probably hold true. I imagine India will be rather strong this century, as well - the US would do well to cultivate relations with that great democracy, they would be an excellent ally to help keep China from getting too big for it's britches. Russia could revive itself, but I have my doubts, as their population is spiraling downwards and they need a larger population to take advantage of all their land and resources. (If Christianity or Islam became much more prominent, their birthrates would probably go up)
Europe will likely be strong, but not quite as strong as people are predicting. It's a shame that they are dragging themselves down with their low birthrates and bad social policies which are negatively impacting their economies, Europe would be an excellent ally to the US.
Some wildcards could be Brazil and the Middle East. Brazil is shaping up to be a regional power capable of maintaining order in South America; another country the US should try to court. They need to deal with some corruption problems, however, and with their deforestation problems. The Middle East is unlikely to be much different in this century than in the last couple of decades, but if, and this is a big if, one nation able to seize control of much or all of the ME, then that could spell trouble. With that much oil, they could cause a lot of problems for a lot of the rest of us if the ruling government wasn't friendly to outsiders, which they probably wouldn't be.
Africa will probably get better, but won't be in any position to challenge the US, Europe, or powers of Asia anytime soon.