MedicineNet - Content-rich commercial site includes interactive groups, ask-the-doctor feature, medical dictionary, comprehensive drug information, medical news, disease-specific information, and links.
Medscape - Searchable commercial collection of full-text articles from such useful sources as the National Institutes of Health and the Centers for Disease Control and Prevention.
Oncolink - Huge collection of cancer information and links based at the University of Pennsylvania. The best starting place found for cancer information.
Medical Matrix - Physician maintained commercial database of annotated health links. Oriented toward medical professionals, but accessible to an educated lay person. A keyword search gets you to a subject index, from which you navigate to the links you want.