The Economic Freedom of the World Index measures the degree of economic freedom in the following five components and averages them out:
1) Size of Government: Expenditures, Taxes, and Enterprises
2) Legal Structure and Security of Property Rights
3) Access to Sound Money
4) Freedom to Trade Internationally
5) Regulation of Credit, Labor, and Business.
See Chapter 1 of the 2007 Annual Report for more detail.
Data is available here.
The sample years include 1970, 1975, 1980, 1985, 1990, 1995, and annually since 1997.
Also see this entry.