The term grey literature refers to research that is either unpublished or has been published in non-commercial form. Many organizations and individuals also provide access to their works online, and most of this material remains unindexed.
Examples of grey literature include:
One of the Google options which can be very helpful is site:
site:health.gov.au - will restrict your search to the Australian Goverment's Department of Health and Ageing web site
You can further focus your search by adding a topic
site:health.gov.au obesity - this will focus on Department of Health and Ageing material on obesity
You can also focus your search to look for specific filetypes
site:health.gov.au obesity filetype:doc - this search will retrieve Department of Health and Ageing .doc files on obesity
site:health.gov.au obesity filetype:pdf - this search will retrieve Department of Health and Ageing .pdf files on obesity
For a list of Google searchable file types see http://support.google.com/webmasters/bin/answer.py?hl=en&answer=35287
Google Advanced Search also allows you to limit your search to a site, or domain, eg. gov.au gov.uk, to a geographical region, to a file type, and to a language. It allows phrase searching in combination with multiple options.