Here are a few meteor impact effect calculators I use. Thought some SciFi GMs might like them. Especially one who wants to throw a major impact into the game.
http://simulator.down2earth.eu/
I like this one because it will show the impact crater over major cities of the world (New York City, London, Cardiff, etc.) using Google Maps.
http://impact.ese.ic.ac.uk/ImpactEffects/
This one takes the numbers you input, then outputs them into a text file listing the numbers of the effects. This one was originally hosted at the University of Arizona. Wished it was totally ported over though because the original used to be several links to further information. I like this one the best because I like gettting the data in a form I can easily printout.
http://www.purdue.edu/IMPACTEARTH
This one is simply a souped up model of the one above using Flash. It's OK, but I still like the upper two better.
rmfr