For those of us who live in the Midwest, here's a Google Maps mashups for tornadoes in the US with both location, force of the tornado and the tornado's path plotted. There's information here on nearly 50,000 tornadoes between 1950 and 2005. You can search by state, county, and size of tornado.

Looking at F5 tornadoes for the entire country, it's pretty obvious where Tornado Alley lies. And also, Missouri appears to get fewer big tornadoes than the states surrounding it.