The Congressional Research Service is a public policy research organization that writes reports for members of the United States Congress. These reports have only recently been released directly to the public, but aren't part of the Federal Depository Library Program. The initial public offering of CRS Reports includes only those published beginning in 2018, although other reports may be added. The Congressional Research Service public site for CRS Reports is available here.
There are a number of public and private organizations that have acquired CRS reports and have released them to the public. This page links to some of these organizations.