Mainstream Health
AMEDEO
Better Sleep.org - Small site that has a lot of useful information on improving an individual's sleep health.
Biome
Center for Disease Control and Prevention
American Council on Science and Health - This is "a consumer education consortium concerned with issues related to food, nutrition, chemicals, pharmaceuticals, lifestyle, the environment and health. ACSH is an independent, nonprofit, tax-exempt organization . . . . .ACSH was founded in 1978 by a group of scientists who had become concerned that many important public policies related to health and the environment did not have a sound scientific basis. These scientists created the organization to add reason and balance to debates about public health issues and bring common sense views to the public."
Drugs.com - Drug information online. Extensive library of info on prescription drugs and their interactions with other drugs. Has a nice "pill identifier" feature, too. If you must take prescriptions, this is a great resource.
Druginfo.net - From the site: "Drug Infonet provides drug and disease information for your healthcare needs . . . . Drug Infonet brings this free resource to you so that you become a more informed consumer of healthcare."
Emedicine
eMedicine Health - From the site: "eMedicineHealth provides comprehensive first aid and emergency information to prepare you for medical emergencies, injuries, minor medical conditions and basic first aid. In addition, you will find health, medical, lifestyle and wellness information for your entire family."
Family Doctor.org
GP Notebook (medical encyclopedia)
Healthfinder.gov
Health Library.com - Indian site, but is in English. Has all sorts of health information on it, including a nice virtual library of e-books. Does have traditional and modern medicine info.
Health.Wiki.com - From the site: "a user powered health community . Our community writes about health issues important to them and contributes to our growing health encyclopedia."
Healthy World Online
How good are your health sources?
Internet Drug Index
Life Extension Foundation
Mayo Clinic - The actual clinic is not available on the web, but their website has a lot of information. Description from the site: "Mayo Clinic's three main Web sites provide information and services from the world's first and largest integrated, not-for-profit group medical practice. Manage your health with information and tools that reflect the expertise of Mayo's 2,500 physicians and scientists."
MedicineNet.com - They claim to bring doctor's knowledge to you. Has health news, disease prevention and general symptomology.
Medline Plus - Sponsored by the U.S. National Library of Medicine and the National Institutes of Health, this site has a wealth of health topics on over 700 diseases. It even appears to take a balanced approach by talking about both supplements and drugs.
Medline Plus Encyclopedia
National Electronic Library of Health
Natural Health Expertise
Online Medical Dictionary
The Quack Files - Site that attempts to debunk some of the alternative medicine myths.
Quack Watch - Reportedly independent site that tries to oust all the quacks in the alternative medical field (and even those in the mainstream that are quacks).
Red Flags Weekly
Remedy Find
RxList - The internet drug list. Has info on all the drugs you may want to look up.
Safe Fetus - If you or someone you love is pregnant, this is a great place to look up specific information about how prescription drugs will affect you or your unborn child.
WebMD - From the site: "WebMD provides valuable health information, tools for managing your health, and support to those who seek information." A trusted name in traditional medical information on the web.
Wrong Diagnosis? - A site with symptoms of over 3000 diseases. They claim to be unaffiliated with any pharma or medical association, which is nice for an unbiased opinion.

Dietary Health
Food Defects Action Level (FDA)
Food Science Central
Nutrition Journal - Freely accessible science journal about nutrition