For effective writing, one has to do extensive research. Physical books and journals are always very valuable peer-reviewed sources. But the internet should also be researched. The internet may be considered as the surface internet/web and the deep internet/web. Standard search engines search only the surface web. However, the deep web/ hidden web/ invisible internet is more than 500 times larger than the surface web. For comprehensive online research, the resources available on both the surface web and the deep web, should be referenced.
Following is a short list of surface web and deep web databases/search engines. They are listed alphabetically.
Surface Web Databases / Search Engines
All Search Engines. http://www.allsearchengines.com/
Virtual Sites. http://www.virtualfreesites.com/
Deep Web Databases / Search Engines
Combined Health Information Database. http://pathmicro.med.sc.edu/chidmic.htm
Directory of Open Access Journals. http://www.doaj.org/
Guidestar Directory of Charities and Nonprofit Organizations. http://www.guidestar.org/NonprofitDirectory.aspx
Human Genome Database. http://web.ornl.gov/sci/techresources/Human_Genome/research/databases.shtml
Internet Sacred Text Archive. http://www.sacred-texts.com/
Plants Database. http://plants.usda.gov/
UCLA Library. http://www.library.ucla.edu/search/databases
University of Michigan Stephen S. Clark Library. http://www.lib.umich.edu/clark-library
Virtual Library. http://vlib.org/
Voice of the Shuttle. http://vos.ucsb.edu/