Here are some sites that contain useful information on various health problems and drugs.
North American Chronic Pain Association of Canada A wealth of information
Dragonrider's Page on Chronic Pain, Chronic Fatigue Syndrome, and Fibromyalgia
Medical Matrix Another great source of information, oriented towards health care professionals
Centerwatch Clinical Trials Information about ongoing or upcoming clinical trials
Centerwatch Drug List Newly approved drugs
Healthtouch Drug information
Medicine Net General medical information
Mediconsult General medical information
Pharmaceutical Information Network General medical information
Rxlist Comprehensive drug descriptions
Talarian Index Index of medical terms with links to information
Usenet sci.med.pharmacy