Makes DMing a bit easier.
http://www.wizards.com/default.asp?x=dnd/dnd/20010320b (http://www.wizards.com/default.asp?x=dnd/dnd/20010320b)
Another one:
http://www.d20srd.org/extras/d20encountercalculator/
Pathfinder version:
http://webpages.charter.net/tedsarah/Pathfinder/utilities/encounterCalculator.htm