The 'free' part isn't what should worry you. Free clinics and street medicine teams have been doing that for a while now. But you are still responsible for everything else in your care as others have pointed out.
As for general knowledge for the masses, this too is possible, but caution is needed. There are a ton of medical blogs out there, not to mention people like Dr. Oz, who give out advice to the masses. The key there is that it's generally benign in nature, not directed at specific patient populations, and usually has some disclaimer that "this is not medical advice. seek a doctor for medical problems." You don't lose freedom of speech just because you have an MD, but you are more likely to be sued.
So in summary: yes you can do it if done right, but start with a legal consult and be cautious.