PubMed is a free service for performing searches on articles in the life sciences and biomedicine.

From the website: "PubMed is a service of the U.S. National Library of Medicine that includes over 16 million citations from MEDLINE and other life science journals for biomedical articles back to the 1950s. PubMed includes links to full text articles and other related resources."

The Google Scholar Web Search Engine can find many freely-available scholarly articles and reports for immediate download in Adobe PDF format. This free utility has similarities to the well-known (pay-only) Web of Science, including the ability to view all works that have cited a given article.