MedlinePlus a comprehensive resource of information about all things medical. MedlinePlus links to reputable sites with additional information. Consider it a good place to start looking for information about a medical topic.
CDC website The US Centers for Disease Control and Prevention is a wealth of information about a variety of health topics with something for healthcare consumers and healthcare providers. A visit to the home page will give you an idea of what is there. One common use of this site is to keep up with infectious diseases. Influenza every year and Zika virus this year (2016).
Mayo Clinic Their patient care and health information materials are very good; worth a look.
HIH Senior Health A website of the National Institutes of Health containing information aimed at older adults.
PubMedHealth A website devoted to Clinical Effectiveness Research. Has a nice section "For Consumers".
OrphaNet A rare diseases database from the European Union.
NORD National Organization for Rare Disorders from the U.S.
DailyMed "This website contains 85636 drug listings as submitted to the Food and Drug Administration (FDA). At the present time, this Web site does not contain a complete listing of labels for approved prescription drugs." For routine drug information I would suggest using MedlinePlus Drugs and Supplements.
LabTestsOnline A nice way to find information about a variety of lab tests. Produced by the American Association for Clinical Chemistry.
Genetics Home Reference A user friendly site which explains genetic contributions to disease. For an example look under diseases and find alpha-1 antitrypsin deficiency. There is a good deal of useful information to be found.
The Medical Library Association Consumer and Patient Health Information Section (CAPHIS) puts together a thorough list of top websites for healthcare consumers. You can find this website here: http://caphis.mlanet.org/consumer/
Provides a simple approach to whether to believe what you are reading on an unfamiliar website.