Grey Source

A project of GreyNet International that has links to a large number of organizations producing and/or processing grey literature

Open Grey Repository 

Access to over 700,000 records of European grey literature but usually does not have links to full-text.

Grey Literature Report

A project of the New York Academy of Medicine that has a database of health-related grey literature.

Policy Archive

A comprehensive digital library of public policy research containing over 30,000 documents created through voluntary uploads of policy documents

Google Scholar

Google Scholar provides some access to scholarly grey literature but its coverage is uneven and hard to predict.