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 (4th International Conference on Grey Literature, 1997)
Technical and committee reports, census data, conference papers and proceedings, theses & dissertations, newsletters, preprints...
Universities, research institutes, industry and government institutions
Web search (Google & Google Scholar)
Specific sites:
Organizations and foundations websites like AAMC, AMEE, and Best Evidence for Medical Education Collaboration (see more in Table 2)