Grey literature is that body of material that is normally published by research organizations, non-governmental bodies, grass roots movements, think tanks, etc. that fall outside the realm of major commercial or academic publishers. Below are several open access and subscription-based databases that provide access to grey literature:
The Bielefeld Academic Search Engine (BASE) indexes academic open access web resources.
Full text working papers, conference proceedings, policy briefs, economic indicators, and other resources in international affairs.
The content in Directory of Open Access Resources is searchable.
Materials harvested by Open Access Initiative.
Downloadable papers and journal articles in political science, economics and finance, law, and other fields.