CDC: Centers for Disease Control
The CDC is part of the Department of Health and Human Services and focuses on issues of public health. This site contains information on diseases, conditions, environmental health, workplace safety and healthy living.
American Heart Association
One of the best websites on heart disease and stroke, the American Heart Association covers all aspects of these issues.
Practical info on many relevant topics with sections for parents, kids, and teens. From the nonprofit Nemours Foundation, providing doctor-approved health information from pre-birth through adolescence.