A website created by Alex Wellerstein and sponsored by the College of Arts and Letters, Stevens Institute of Technology, called NukeMap, lets you calculate the blast radius, fallout, casualties and a host of other terrifying stats for nearly all nuclear weapons currently in existence.
You select where on the map you want to be ground zero (of course most people are going to use their hometown to see what happens).
You can select from a list of known, existing nuclear ordnance, or enter your own yield.
Then you can turn on/off a variety of options to see fireball radius, blast radius, nuclear fallout and several other options. The legend tells you more info about each zone.
The mapping software is powered by Google Maps.
Here is a sample detonation showing what would happen if a Chinese ICBM hit the city of Chicago.