A Google Custom Search Engine maintained by the Harvard Kennedy School's Library and Knowledge Services unit. Covers 1200+ partisan and non-partisan think-tanks. US-centric but still a valuable tool for finding reports and content published by these organizations.
A searchable, browsable directory of policy produced by think-tanks and other organizations. Includes a list of organizations as well as the Custom Search Engine behind this. Maintained by the University of Oregon Libraries.
Maintained by Michigan State University's Institute for Public Policy and Social Research since 2011. Can search by keyword or filter. Abstracts include a summary of policy implications. Covers a broad range of policy research domains and research types.