In his book, Machiavelli detailed how to gain and maintain political power, through whichever means necessary. He lays out desired characteristics for princes (political leaders) including stinginess, cruelness, breaking promises if they aren't in their own best interest, taking actions that might not be morally acceptable in the interest of the 'greater good', and taking on great projects to gain in popularity among others. He laid out the gap between religious and ethical values and the actions often necessary to lead. This gap and the debate around state ethics and moral responsibility has remained a large source of debate and study into the modern era. Examples include the current debate in the United States about drone attacks and what constitutes a necessary evil for the good of the nation and what goes too far ethically into criminal behavior. These discussions and arguments that Machiavelli started with his work, are still ongoing in most corners of the globe.
Aside from the fact that his book "The Prince" is still widely read, cited, and applicable, perhaps the greatest impact that Machiavelli has had on society is that his name inspired a word still used today when talking about someone who is manipulative, deceitful, or scheming: Machiavellian.