American Society for Nutrition. Great organization where you can get involved with meetings. You can also donate to nutrition based research.
Healthy Schools Campaign. In an effort to get healthier food in schools and unhealthy food out. You can donate, sign petitions, and volunteer time.
Action Against Hunger. This is an international organization that raises money to support projects in 3rd world countries.
GAIN Health. I’ve also posted this under related links. Good organization to get involved internationally with nutrition.
Children’s Health. Talks about helping ensure that your kids have the nutrition they need. You can donate to research.