Tblkjack, Alternative Prime, and Gary Rowell -- I would have to agree. Those three have each made an impact on the modern world, each in their own way.
Butch News and Tusitala Tom -- In regards to religion, I would combine both of your answers. Those who have been named "prophets" have caused more hatred and war in the world than anyone, but I believe that is only due to the reactions of their followers and adversaries.
Jan -- That's exactly why I asked it! It allows everyone to open up their minds and provide their true opinions. There are SO many historical figures that could be named, and I'd love to read everyone's thoughts. Plus, it could be a great place to create some civilized debate. :) Keep 'em coming!