GAO Reports: The U.S. Government Accountability Office (GAO) is an independent nonpartisan agency that works for Congress. Often called the "congressional watchdog," GAO investigates how the federal government spends taxpayer dollars. The work of the GAO is done at the request of congressional committees or subcommittees or is mandated by pubic laws or committee reports. They also undertake research under the authority of the Comptroller General. They support congressional oversight by: (1) auditing agency operations to determine if federal funds are spent efficiently & effectively; (2) investigating allegations of illegal or improper activities; (3) reporting on how well government programs & policies are meeting objectives; (4) performing policy analyses and outlining options for congressional considerations; and (5) issuing legal decisions and opinions.
Committee on Ways an Means House of Representatives Green Book: The Green Book has become a valuable resource and standard reference on American social policy. It is widely used by Members of Congress and their staffs, analysts in congressional and administrative agencies, members of the media, scholars, and citizens interested in the Nation's social policy.
Congress.gov: This site, formerly known as Thomas.gov, from the Library of Congress provides legislative information including bill tracking, presidential nominations, the text of committee reports, and other information.
Congressional Research Service (CRS) Reports: The Congressional Research Service (CRS) is a division of the Library of Congress. They produce analytical, non-partisan reports on topics of interest to members of Congress. In 2018 they stood up a public website for dissemination of their more recent reports, and there are a number of free and subscription sites where you can find current and older CRS reports. You can also request copies from your Congressional Representative.
Center for Strategic & International Studies:
An international policy institute, the CSIS focuses on a number of topics including defense and security, governance, technology and global trends.
Policy Archive: http://www.policyarchive.org
This collection of public policy research is gathered from sources such as the American Bar Association, the Brookings Institution, the Ford Foundation, the Institute for Policy Studies, and others.
RAND Institute: http://www.rand.org
The RAND institute provides research in a variety of areas such as arts, civil justice, education, national security, public safety, and other areas.