Headed by a group of 12 doctors, dietitians and PhDs,
The Physicians Committee for Responsible Medicine is dedicated to promoting a healthy diet as a form of preventative medicine and performing clinical research to share with the medical community the benefits of a plant-based diet. They also promote alternatives to animal-based medical research. As far as I know, they are the only group of their kind. Even if you don't decide to donate to them, check out their website for a plethora of health and nutritional information!
More vegetarian charities