Grey literature is information produced outside of commercial publishing and distribution, including:
Grey (or gray) literature can be more current than traditional sources, but since it does not go through the peer-review process the quality may be variable and all sources must be critically evaluated. Researching grey sources can help identify a gap in the literature, and may offer a fuller picture and/or balance possible biases in commercial publications.
AHRQ Report: Finding Grey Literature
Medline Plus Organization Directory: search individual organizations of interest for reports, conference proceedings, newsletters, guidelines, etc
WHO: World Health Organization: reports and guidelines on public and global health topics
Tricks for finding unpublished material in Google and Google Scholar