These sites are in no particular order, and I don't necessarily personally agree with everything on every site. But I find them valuable for overall knowledge and as sources of information, as well reading the experiences of others who have a special interest in avian nutrition.

Parrot Diet

Diet and Feather Picking

Psittacine Bird Diet Recommendations by by C. Newman, PhD, DVM

Avian Nutrition 101, courtesy of the Oklahoma Cage Bird Society

Feeding Your Bird for Health by David McCluggage, D.V.M., C.V.A.

Avian Nutrition by Dr. Adrian Gallagher

Seed vs. Pellets

The Problem with Pellets

Diet and Nutrition

What's in a Pellet?

Benefits of a Base Diet - Foods for a Healthy Bird

Feeding Our Birds is part one of a very informative article; part two is A Healthy Diet.

The Greening of the Parrot Diet

Pellets or Supplements?

The Importance of Phytonutrients

Parrot Recipes

Bird Recipes

Hope for Libby and Will

African Grey Myths: Greys Need More Calcium

What's the Difference Between a Sweet Potato and a Yam?

Sweet Potatoes

Feeding the Companion Parrot

Parrot Diet from Thomas River Aviary

Calcium Metabolism in Birds

And finally, Naturally Healthy Pet is the site of the late Alicia McWatters, PhD, CNC, a nutritionist who dedicated her life to the study of nutrition issues in our pets, including extensive and intensive study of avian nutrition. Even if you haven't jumped on the organic, all-natural bandwagon, there are lots of facts about the subject as well as opinion; very valuable stuff! There are many articles there of great interest, but some of my favorites are:
Plant Enzymes: Providing Optimum Avian Health
Parrot Diet
Millet: Is It Nutritious for my Birds?
Are Sunflower Seeds For The Birds?
Why Food is Better Than Pellets
Synthetic Bird Feeds: Do They Promote Health, or Disease?
The Truth About Vitamins