Grey literature exists outside of and is uncontrolled by the usual academic and commercial publishers. It includes conference proceedings, occasional and working papers, policy papers and briefs, hearings, reports, and more.
National Bureau of Economic Research (NBER)
A "private, non-profit, non-partisan organization dedicated to conducting economic research and to disseminating research findings among academics, public policy makers, and business professionals. NBER-affiliated researchers study a wide range of topics and they employ many different methods in their work. Key focus areas include developing new statistical measurements, estimating quantitative models of economic behavior, and analyzing the effects of public policies."
CGS Policy Archive-- collects and archives public policy research publications from US think tanks.
NIRA World Directory of Think Tanks-- indexed by country with descriptions, contact information, and links to organizations' websites.
WWW Virtual Library: Research Institutes-- list of think tank-related links and resources maintained by Wayne Selcher at Elizabethtown College
Public Policy Institutes-- a Yahoo directory of important international think tanks with links to organizations' websites.