Think Tank Search (Harvard Kennedy School) has combined a directory of U.S. and international think tanks with a customized Google search 0f 250+ think tank web sites.
Think Tanks and Civil Societies Program (International Relations Program, UPenn), is a site examines the role, history, and purpose of think tanks including research about think tanks. Data is broken down by region, issues, trends, etc.
Think Tank Watch is a searchable database of policy research and analysis.
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