Online version, with features not available in the print version, and "continuously updated to ensure that the information is as up-to-date as possible."
National Library of Medicine - National Institutes of Health launching point for health sciences information, from peer-reviewed literature to clinical trials. Be sure to look at the quick start guide, information on finding the full text of articles, and the tutorials. Also check out the citation matchers, useful if you have incomplete information about an article.