Murray Newton Rothbard (March 2, 1926 – January 7, 1995) was an American philosopher, individualist anarchist author and economist of the Austrian School who helped define modern libertarianism and popularized a form of free-market anarchism he termed "anarcho-capitalism". Rothbard wrote over twenty books.
His 1973 book For a New Liberty is well-known as perhaps the essential primer on libertarian political views. His 1962 book Man, Economy, and State is also well-known. The Ethics of Liberty, his 1982 book is regarded as a tour-de-force of libertarian morality. His books are still in print and available from the Mises Institute.
Rothbard's views always remained consistently libertarian even as his political alliances shifted over the years. He was part of the Old Right, the New Left, then the Libertarian Party and finally part of the paleoconservative movement just prior to his death. In many ways he defined libertarianism as a distinct political philosophy.
 Rothbardianism and Left-Rothbardianism
Rothbardianism has gained some popularity as a self-identifier by those libertarians who hold to Murray Rothbard's particular combination of libertarian views: Austrian School economics, antiwar and antimilitarist, hard money views on banking and currency, a natural rights based morality, favoring complete abolition of the state while supporting political activity including voting, and viewing the Old Right and New Left as natural allies of the libertarian cause. This is to distinguish Rothbardian libertarians from those espousing other conceptions of libertarianism, including Milton Friedman's Chicago School economics, minarchists, anti-political anarchists who oppose voting, those favoring consequentialist conceptions of morality or rejecting morality altogether, and "neolibertarians" who support U.S. military interventionism abroad.
A "Left-Rothbardian" is somebody who identifies most closely with Murray Rothbard's period of alliance with the New Left and views libertarianism in general, and Rothbardian libertarianism specifically as part of the political left rather than the right. There are also some Rothbardians who prefer to be part of alliances with the political right, usually the paleoconservative movement; see for an example of the latter, Antiwar.com.