Grey literature refers to materials which are not easily found via normal sources such as subscription databases or publisher web sites, but may still contain valuable information for researchers.

There are several sources of grey literature available.

The main organization is Greynet. The Grey Literature Network Service was founded in 1992. The goal of GreyNet is to facilitate dialog, research, and communication between persons and organizations in the field of grey literature.

Scirus is the biggest and the best source for conferences and papers; its part of Scopus. It has a limit for conferences (and also for patents). It describes itself as having 4.4 million conference papers from proceedings and journals.

The British Library Catalogue has a preset limit for conference proceedings that make it easier to find papers from conferences. It contains a large numbers of conference proceedings since it's collection is very broad in scope.

The National Library of Medicine in its catalog – Locator Plus - has different access points you can use to make it easier to find conference reports. If you "Or" the following search terms as one search statement and then pair it with a subject heading in another search statement, you can be pretty precise in locating materials. The same search can be used in PubMed as well: Congresses [pt] OR Congresses as topic [MeSH] OR Overall [pt] OR Consensus development conference [pt] OR Consensus development conference, NIH [pt] OR Consensus development conferences as topic [MeSH] OR Consensus development conferences, NIH as topic [MeSH]

Open SIGLE (System for Information on Grey Literature in Europe) is another source of European origin. At this point it mostly retrospective from 2005 and older, but it states as a goal to get the newer material. And it now contains the conference proceedings and papers from International Conference Series on Grey Literature from 1995-2009 (with newer material to be added shortly).

Worldwidescience.org is another source with some free full text .

OAIster.org is another open source initiative project. It is contained within OCLC WorldCat searches but is also available in a free version.

CADTH "Grey Matters" is a new online source for grey literature searching from Canadian Agency for Drugs and Technologies in Health. It's similar to Cochrane Reviews, but for Canada, and they've made a searchable database of their sources for reports. Grey Source Index is a large listing of sources for grey literature with a subject taxonomy to give you access in all the different subject areas. The idea is to give you further sources to look for more information.