AllRefer.com
A virtual medical encyclopedia offering "extensive information on over 4000 topics arranged in an A-Z order". It is not only extensive but offers easy to navigate pathways and guides to symptoms, diseases, diet & nutrution, health news and more.
BioMedSearch.com
A huge and constantly expanding database for all things health/medicine related. "BioMedSearch is a biomedical search engine that contains NIH/PubMed documents, a large collection of theses, dissertations, and other proprietary publications not found anywhere else for free, making it one of the most comprehensive and powerful free biomedical searches." [Thank you, James!]
Health Oasis
From the Mayo Clinic, "Reliable Information for a Healthier Life".
Shape Up America!
Created by former US Surgeon General C.Everett Koop, an online fitness center which offers diagnosis and fitness plans customized to individuals who complete a brief questionnaire. Includes practical suggestions for exercise and diet.
Medicine & Medication
Clinical Trials
A "clinical trial listings service" from Centerwatch. Resources for patients and health professionals.
Merck Manual
The centennial edition of this respected manual "dedicated to providing reliable, easy-to-use medical information".
RxList
"The Internet Drug List". Created by a pharmacist. Features search tools, current drug information, and more. (Sadly, they've recently lost their very good comix page - all biz now.)