MedlinePlus
"MedlinePlus has extensive information from the National Institutes of Health and other trusted sources on over 700 diseases and conditions. There are also lists of hospitals and physicians, 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."
The link address is: http://www.nlm.nih.gov/medlineplus/