A great many people have contributed to the study of economics, yet the they have not all made equal contributions. I will highlight here those who have specifically contributed to the theory and practice of free markets.

Carl Menger

Many consider Carl Menger the father of The Austrian School of Economics
Biography of Carl Menger

Ludwig von Mises

Ludwig von Mises received credit, in the minds of many, for the resurrection the Austrian school of economics.

Murray N. Rothbard

Murray Rothbard, who participated in the seminars of Ludwig von Mises for many years, help to further develop the principles and theories espoused by Mises.
Biography of Murray Rothbard

Eugen von Böhm-Bawerk

Böhm-Bawerk linked the intertemporal structure of production to the intertemporal preferences of workers and other income earners.
Biography of Eugen von Böhm-Bawerk

Henry Hazlitt

Henry Hazlitt was a journalist by trade, but he must be listed amongst economists because he made economic theory understandable for many who did not formerly understand it.
Biography of Henry Hazlitt

And More:

Juan de Mariana
Richard Cantillon
A.R.J Turgot
Jean-Baptiste Say
Fredric Bastiat
Philip Wicksteed
Frank A. Fetter
F.A. Hayek
William H. Hutt
Wilhem Ropke