MedlinePlus is the best for my purposes and aggregates all sorts of information and links to other sites.

http://medlineplus.gov/

About MedlinePlus

http://www.nlm.nih.gov/medlineplus/a...dlineplus.html

MedlinePlus will direct you to information to help answer health questions. MedlinePlus brings together authoritative information from NLM, the National Institutes of Health (NIH), and other government agencies and health-related organizations. Preformulated MEDLINE searches are included in MedlinePlus and give easy access to medical journal articles. MedlinePlus also has extensive information about drugs, an illustrated medical encyclopedia, interactive patient tutorials, and latest health news.

MedlinePlus Guide to Healthy Web Surfing
http://www.nlm.nih.gov/medlineplus/h...ebsurfing.html