Medline Plus - authoritative health and drug information from the National Library of Medicine and the National Institutes of Health. The site also includes a medical encyclopedia and dictionary, interactive tutorials, information about clinical trials and a directory of doctors, dentists and hospitals throughout the US.
WebMD - Health, first aid and lifestyle information. This site includes a pill identifier as well as descriptions of medical tests and tools. (Note that this is a commercial site and it does include advertisements. For best results, you should compare information from this site with information from Medline Plus .)
PubMed - a service of the U.S. National Library of Medicine that includes over 18 million citations from MEDLINE and other life science journals for biomedical articles back to 1948. PubMed includes links to full text articles and other related resources.
Lab Tests Online - a peer-reviewed, non-commercial resource produced by the American Association for Clinical Chemistry. This site provides information about clinical lab tests ranging from pre-test preparation to description of procedure and interpreting test results. You may search for tests by keyword or browse an index of texts by name, condition being tested, or target age group.