American Association of Retired Persons (AARP) Public Policy Institute
Congressional Research Service (CRS) - the public policy research arm of the US Congress. It does not make its reports available to the public, but they do not prevent members of Congress from giving them out individually and some government agencies from posting reports on the web. They are available through several sources:
National Governor's Association - includes an index to state documents on public policy and welfare issues with links to the on-line text.
PEW Research Center for the People and the Press
Public Policy Institute of California