WebMD has very good consumer-class content related to heart anatomy, physiology, conditions and treatments. This is a good site for the non-professional to obtain a solid foundation to enable search and comprehension of professional
Dr. Larry Creswell's blog on the topic of athletes and heart health. Dr. Creswell is a multi-sport endurance athlete who is a faculty member at the University of Mississippi School of Medicine. His searchable blog is interesting and informative.
Khan Academy has some excellent video lectures describing cardiovascular physiology and various disease states, covering coronary artery disease, valve disease and rhythm abnormalities.
The “Plant Positive” blog has a interesting content regarding the benefits of a plant-based diet. It is not a “medical” site, but nevertheless provides very good insight to the evolving school of thought that advocates a vegan diet for optimal health.
Medscape is a site with content for medical professionals that requires registration, which is free (you do not need to be a licensed health professional). It has content related to a variety of medical conditions as well as comprehensive drug information.
American College of Cardiology CardioSource provides a search engine for professional publications. Many documents require journal subscriptions, but search results are returned with an “unlocked” icon that indicating those which may be viewed without charge.