In general, grey literature publications are non-conventional, fugitive, and sometimes ephemeral publications. They may include, but are not limited to the following types of materials: reports (pre-prints, preliminary progress and advanced reports, technical reports, statistical reports, memoranda, state-of-the art reports, market research reports, etc.), theses, conference proceedings, technical specifications and standards, non-commercial translations, bibliographies, technical and commercial documentation, and official documents not published commercially (primarily government reports and documents) (Alberani, 1990).
Just as we evaluate web resources, it's important to evaluate grey literature material found on the web and consider the source publishing the material.
Grey literature is self-published and typically comes from: Government agencies, research institutes, organizations/companies, associations. (What is the Value of Grey Literature?)
Some "new" sources of grey literature: pre-prints, blogs (Padagogy Wheel), “front matter” preliminary research results, institutional repositories (IRs) (Value of Grey Literature to Scholarly Research in the Digital Age)