Hurricanes spin counterclockwise (like all low pressure centers in the northern hemisphere) because of the Coriolis Effect. Because the equator rotates faster than other areas of the Earth's surface, anything moving in a straight line on a North to South axis will eventually curve. If you could throw a ball hard enough and far enough, it too would eventually curve.
Since air flows into a low pressure center, sort of like water going down a drain, it curves counterclockwise. Conversely, since air flows out of a high pressure center, they rotate clockwise above the equator, and counterclockwise down under.
This phenomenon was first observed by French mathematician Gaspard Coriolis in the early 1800's. The Coriolis effect must also be taken into account when the trajectory of a rocket is calculated.
