Back in my college days I did a report calculating the effects of a 20 megaton nuclear bomb being detonated over Tinker Air Force Base near Oklahoma City. A new web mash-up makes that sort of thing terribly easy these days. Ground Zero from CarlosLabs lets you detonate a variety of atomic and thermonuclear devices over any spot on Earth and see the effects. They even include asteroid strikes. The BLDG Blog has an interesting article on this.