Medical, health, and well-being sites abound on the Web. Most of them are private sites by doctors and other practitioners, but some of the best of the major "hubs" are listed below. Most of these feature doctor-hosted chats and/or message boards, support groups for various diseases and medical conditions, news, and physician directories. Many feature online prescription refills and access to products. The e-commerce side of these sites will be the quickest to expand. 

AllHealth.com (a feature of the new iVillage portal, and based on a well-known print handbook)
www.allhealth.com/

Alternative Medicine Home Page
www.pitt.edu/~cbw/internet.html

America's Doctor (features one-on-one chats with real doctors, although these chats are sometimes unsatisfactory)
www.americasdoctor.com/

American Medical Association (much more user-friendly than previous iterations of this site, with lots of useful content; an excellent way to find a local doctor)
www.ama-assn.org/

APA Health Center (info about psychology and area psychologists)
helping.apa.org/

Ask Dr. Weil (site of the author and alternative practitioner Dr. Andrew Weil; well-structured and informative)
www.drweil.com/

Benefits CheckUp (find out about medical benefits available to older US citizens)
benefitscheckup.org

Calorie Counter (keep track of those calories)
benefitscheckup.org

Cancer Page (large informative site on cancers, research, their cures, FAQs, along with their Ask-a-Nurse, chat, and message board features)
www.cancerpage.com/

CancerNet (excellent resource, offshoot of the National Cancer Institute)
www.cancernet.gov/

CBS HealthWatch (lots of good content, including drug directories, medical test handbooks, self-care handbooks, and guides to managed care)
www.cbsnews.com/sections/health/main204.shtml

Center for Disease Control
www.cdc.gov/

CenterWatch (keeps track of clinical trials being conducted in 20 disease categories, a summary of the research, and contact info)
www.centerwatch.com/

Discovery Health (good site, slanted towards non-technical users)
www.discoveryhealth.com/

Doctorline (large, informative database of doctors)
doctorline.com/

Doctorpage (another, less well-organized doctor database)
www.doctorpage.com/

Dr. Koop's Community (huge database of medical information, a large drug-interaction database, links to lots of online pharmacies, expert chats, and more; now allied with AOL and Drug Emporium)
www.drkoop.com/

DrSpock (child-care site based on the teachings of the legendary Dr. Spock)
www.drspock.com/

DrugDigest (health and drug info)
www.drugdigest.org/

Drugstore.com (an online pharmacy)
www.drugstore.com/

eHealth Insurance (plain-language comparisons and explanations of the wide variety of health insurance options available)
www.ehealthinsurance.com/ehi/index.html

Epilepsy Foundation
www.efa.org/

FitDay (plan your fitness regimen)
www.fitday.com/

Getting Well Network (summaries of illnesses and diseases; includes a drug reference and a list of current clinical trials)
www.gettingwell.com/

Health Central (online home of radio doc Dean Edell)
www.healthcentral.com/

Health Facts
www.nih.gov/

Health Network (part of the Fox Network group; provides live Webcasts of ongoing surgeries for the strong of stomach; easy to use and navigate)
www.thehealthnetwork.com/

Health Square (medical site oriented to women and family issues)
www.healthsquare.com/

Healthfinder (a U.S. government-sponsored site, lots of good content)
www.healthfinder.gov/

Healthgrades (large site that lets you search for local doctors, as well as check them out for board certification and possible sanctions)
www.healthgrades.com/

HealthSCOUT (medical site featuring a search/definition feature: type in, say, "autism," and get back a definition and a set of relevant links)
www.healthscout.com/

Healthy Ideas (affiliated with Women.com)
www.healthyideas.com/

HealthyPet Hospital Finder (find a vet or animal hospital in your community)
www.healthypet.com/hospital_search.aspx

Intellihealth (linked to Johns Hopkins University and Aetna; excellent Health News and Ask the Doc sections; tons of information)
www.intellihealth.com/

KidsHealth for Kids (good site for medical and related info related to childhood)
www.kidshealth.org/

Mayo Clinic (excellent medical reference site)
www.mayoclinic.com/

MedicineNet
www.medicinenet.com/ 

MedScape (primarily for medical professionals; large professional library available for purchase)
www.medscape.com/

Merck Manual of Medical Information - Home Edition (translate medical jargon)
www.merckhomeedition.com/

MyHealthScore (allows you to research the cost of various medical procedures and how hospitals calculate charges)
www.myhealthscore.com/

National Institute of Health Gateway Search (gateway site to the online resources of the federal government and its agencies)
gateway.nlm.nih.gov/gw/Cmd/

National Library of Medicine (huge selection of medical literature)
www.nlm.nih.gov

Olen Pregnancy, Ovulation, and Childbirth Calendar (keep up with the progress of your pregnancy)
www.parentsplace.com/pregnancy/calendar/

PDR.net for Consumers (gateway to medical resources hinging on the info available in the Physician's Desk Reference volumes)
consumer.pdr.net/

Physician.com (another database of medical practicioners)
www.physician.com/

Prescriptions and Health (free, detailed medication information)
www.merck-medco.com/medco/index.jsp

PubMed (searchable database of medical journal articles)
www.ncbi.nlm.nih.gov/entrez/query.fcgi

Quackwatch (site that debunks health myths, fallacies, and frauds)
www.quackwatch.com/

Red Cross
www.redcross.org/

RXList (online drug index)
www.rxlist.com/

SleepChannel (find out about sleep, sleep disorders, and more)
www.sleepchannel.net/

Thrive Online (part of the women-oriented Oxygen network)
thriveonline.oxygen.com/

Video Fitness (online exercise and fitness site)
www.videofitness.com/

Virtual Hospital (U. of Iowa site; lots of info about diseases and hospital/surgical procedures)
www.vh.org/

WebMD (large, varied medical site affiliated with CNN and Healtheon, very user-friendly if somewhat overwhelming to the casual user; large section of moderated, expert-led chat sessions)
www.webmd.com/