I believe the United States is an imperialist power, but probably one of the least terrible and crushing forms that has come into existence.
Unlike most other empires; we tend to not use our military muscle to enforce our will. That may sound strange given our current situation, but given our massive capability and the territory we seek to control we have usually done the controlling through mostly indirect means.
Earlier empires would take on a hostile territory or revolting province by invading, raping and murdering as many people as they could catch, burning the village or town down, and salting the lands. (We'll use Carthage as a wonderful example of such forms of imperialism).
I'd like you to take note what the Huns did to Eastern Europe when they marched across the continent. The Romans had quite the time dealing with entire regions evacuating into their territory.
The United States, though, ended its genocidal tendencies after the Native Americans were gone and present-day America was founded. If anything, we were far more imperialistic in the past than now; and far more violent and destructive.
Nowadays, we are content to bully, spy, propagandize, economically blackmail, and coup our way towards empire. Our list of nations we've directly attacked (after WWII) are small: Iraq(2), Afghanistan, North Korea, North Vietnam, and Serbia.
With regards to Israel (which always seems to come up whenever U.S. imperialism is discussed), the matter is much more complicated than either side is willing to admit. At any rate; it is an indirect form of imperialism at the most.
We do, of course, have indirect control over many nations in Central and South America (but that grip seems to be slipping more and more). Africa is somewhat under our control; and we've engaged in some rather underhanded dealings with regards to Somalia when we helped their neighbors overthrow their popular government.
Asia is somewhat in our control as well, but that is slipping as regional powers such as China grow in importance and as Japan and South Korea become more and more fed up with U.S. presence.
We maintain military bases around the world, from Europe to Asia. But in Europe they seem to be mostly for show; a political bargaining chip at the most when dealing with Russia.
The Middle East is a strange place, our bases there seem to do two things:
1. Make our oil cheaper than the Europe's.
2. Destabilize the region more and more.
Our control over the Middle East slips by the hour, if not the minute; we certainly have far less control over that region than past empires (from the British to the Ottomans).
For the most part, our empire is built to do two things:
1. Make us look big and scary so we can feel good about ourselves (and to help as a bogey-man at negotiating tables)
2. Open up trading opportunities and continue the market for arms sales and military personelle.