For those of you with a morbid interest in just how bad an asteroid strike could be (see our post Life in the Shooting Gallery for our chances of being hit), the Lunar and Planetary Laboratory at the University of Arizona has an Earth Impact Effects Program, allowing you to calculate just how big the fireball, crater, and other effects would be. No fancy graphics, just cold numbers and text.
Example: I live about 30 miles outside of San Francisco. If a 10 meter porous rock asteroid were to hit there, with a strike energy of about 27 kilotons of TNT, it would leave a crater of 1192 feet, I'd feel ground shaking of about 3.5 on the Richter scale, and the sound of the boom would be about 40 dB. The average intervel between impacts of this size is 6.8 years (fortunately, not all on San Francisco).
Universe Today has the details on the simulator.
Oh, this will be fun to play with.