MedlinePlus (http://medlineplus.gov/) is a resource that is provided by the National Institute of Health and produced by the National Library of Medicine.
According to the web site:
Health professionals and consumers alike can depend on it for information that is authoritative and up-to-date. MedlinePlus has extensive information from the National Institutes of Health and other trusted sources on over 950 diseases and conditions. There are directories, a medical encyclopedia and a medical dictionary, health information in Spanish, extensive information on prescription and nonprescription drugs, health information from the media, and links to thousands of clinical trials. MedlinePlus is updated daily and can be bookmarked at the URL: http://medlineplus.gov/. There is no advertising on this site, nor does MedlinePlus endorse any company or product.