The Fourth International Conference on Grey Literature (GL '99) in Washington, DC, in October 1999 defined grey literature as follows: "That which is produced on all levels of government, academics, business and industry in print and electronic formats, but which is not controlled by commercial publishers."
Types of Grey Literature
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). [copyright NYAM]
It is helpful if you know which bodies publish in your area of research interest, but if you don't know where to look, you could check with your research supervisor, check your subject LibGuide, or use the advanced search features in Google Scholar or Google which allows you to filter to specific website domains for example .gov or .edu
Sources of Grey Literature - some examples
BASE is one of the world's most voluminous search engines especially for academic web resources. BASE provides more than 140 million documents from more than 6,000 sources. You can access the full texts of about 60% of the indexed documents for free (Open Access). BASE is operated by Bielefeld University Library.
Aggregating the world’s open access research papers
System for Information on Grey Literature in Europe, is your open access to 700.000 bibliographical references of grey literature (paper) produced in Europe and allows you to export records and locate the documents.Examples of grey (gray) literature include technical or research reports, doctoral dissertations, some conference papers, some official publications, and other types of grey literature.OpenGrey covers Science, Technology, Biomedical Science, Economics, Social Science and Humanities.The site includes preprints from the GL conferences (GreyNet International) in full text.
A Selection of Web-based Resources in Grey Literature. GreySource provides examples of grey literature to the average net-user and in so doing profiles organizations responsible for its production and/or processing. GreySource identifies the hyperlink directly embedded in a resource, thus allowing immediate and virtual exposure to grey literature.
The web-based resources appear within categories derived from the COSATI (American) and SIGLE (European) Classification Systems, which were recently adapted by the OpenGrey Repository. The few changes that have been introduced into the classification scheme are intended to facilitate the search and retrieval of net-users. New examples are welcome and will be indexed in GreySource.