A little online application that's actually a bit scary.
Did you ever wonder what would happen if a nuclear weapon was detonated over your home... Try this out -- everything from the smallest nuclear device to the largest ever detonated. Put the marker over your city and hit DETONATE. I tried it out later I upped the ante to a 1000x the TSAR bomb to simulate an asteroid hitting the earth.

http://www.nuclearsecrecy.com/nukemap/