Most tornadoes spin counter-clockwise in the northern hemisphere (north of the equator) and clockwise in the southern hemisphere (south of the equator).

It is rare for a tornado to spin clockwise in the northern hemisphere, but it has been observed on occasion.

The Coriolis Effect is what causes hurricanes to spin in different directions in the northern and southern hemispheres.

What this really means, is that when something (like wind) appears to travel in a straight line, the rotation of the earth, will cause it spin in a circle, thereby causing a tornado.  Of course, for a tornado to form, it must also have very, very powerful winds.