Pubmed Central
http://www.pubmedcentral.nih.gov/

"PubMed Central (PMC) is the U.S. National Institutes of Health (NIH) free digital archive of biomedical and life sciences journal literature."

Google Scholar
http://scholar.google.com/

"Google Scholar enables you to search specifically for scholarly literature, including peer-reviewed papers, theses, books, [medical] journals, preprints, abstracts and technical reports from all broad areas of research."

The APA Style Guide to Electronic References (click the icon)