Core

• PubMed
• Cochrane Library
• EMBASE
• Scopus
• Web of Science
• Conference Papers Index

 Specialty

• World Health Organization Databases
• Indian Medlars Centre
• PsycINFO
• ERIC
• CINAHL (Cumulative Index to Nursing and Allied Health Literature)
• Social Science databases
• Business databases

Sources for Finding Databases

• Ulrichs International Periodical Directory [look up individual journals within the discipline or from search results; check in Ulrich's for databases that index the journal]
• Gale Directory of Databases
• Special Libraries lists of databases
• Resource/Research guides in specific subject areas

Citation lists of retrieved articles

Handsearching of key journals

• US Cochrane Center - Resources for Handsearchers

Personal communication with experts in the field

General

• Grey Matters: a practical search tool for evidence-based medicine
  A checklist of all grey literature sources in a Word document
• GreyNet - Grey Literature Network Service 
  [look at the greysource index - link on left]
• Grey Literature Web Conference Series
  An overview of searching for the grey literature from Academy Health
  
• NIH RePort
  Reports, Data, and Analyses of NIH Research Activities
• OAIster
  Contains records of digital resources from open-archive collections worldwide. Free through OCLC.
• Dissertations

Clinical Trials/Results Databases

• Current Controlled Trials
• Clinicaltrials.gov
• WHO ICTRP
• CenterWatch
• IFPMA Clinical Trial Results Portal
• Trial Registries - Duke LibGuide: Developing Systematic Reviews for Comparative Effectiveness Research

Conference Proceedings

• Web of Science Conference Proceedings
• NLM Gateway Meeting Abstracts
• Conference Papers Index

Other Library Guides

• Resources for Searching the Grey Literature
  from Duke University
• Grey Literature Part I
  from the UBC Health Library Wiki
• Grey Literature Part II
  from the UBC Health Library Wiki

Armstrong R, Jackson N, Doyle J, Waters E, Howes F. It's in your hands: the value of handsearching in conducting systematic reviews of public health interventions. J Public Health. 2005;27(4):388-391.

Helmer D, Savoie I, Green C, Kazanjian A. Evidence-based practice: extending the search to find material for the systematic review.  Bull Med Libr Assoc. 2001 Oct;89(4):346-52. PMID:11837256; PMCID: PMC57963

Hopewell S, Clarke MJ, Lefebvre C, Scherer RW. Handsearching versus electronic searching to identify reports of randomized trials. Cochrane Database of Systematic Reviews 2007, Issue 2. Art. No.: MR000001. DOI: 10.1002/14651858.MR000001.pub2.

Hopewell S, McDonald S, Clarke M, Egger M. Grey literature in meta-analyses of randomized trials of health care inventions. Cochrane Database of Systematic Reviews 2007, Issue 2. Art No.: MR000010. PMID: 17443631

Information Service, Centre for Reviews and Dissemination. Finding studies for systematic reviews: a resource list for researchers. Centre for Reviews and Dissemination, University of York, July 2010.