PubMed, a service of the U.S. National Library of Medicine, contains over 17 million citations from MEDLINE and other life science journals. Coverage goes back to the 1950s. PubMed also includes links to full-text articles and MeSH subject headings.
Broad science database including medical science amongst others. Search includes all the PubMed articles, as well as many others. Offers a unique citation finder tool for tracking and finding citations from an article.