PubMed
A publicly accessible database provided by the National Institutes of Health, this allows users to search for biological information using a variety of categories.
Science.gov
Provides access to authoritative, selected science information from U.S. Government agencies, including research and development results.
Google Scholar
Covers many disciplines and sources including articles, theses, books, abstracts, and court opinions from academic publishers, professional societies, online repositories, universities and other web sites.