Hair Nutrition 
I was asked at work what one would take to improve hair growth. I answered with what I remembered, but will add to it some info here.
Hair itself is dead and made from Keratin, the same protein structure that makes up skin and nails also (see here). See a animation of the structure of hair here.
The process of making hair can bee seen here. A really great source of info on the structure of hair is from P&G.
Protein-building block of Keratin
So Proteins are the essential ingredients of hair. Other nutrition however is needed to promote the processes of making hair, keeping hair, and strengthening hair.
1.) Eating Proteins is the first step to growing stronger, longer, faster growing hair, skin, and nails. Too low of proteins, and hair will thin, and slow down growth.
Amino Acids are the building blocks of Proteins. Animal meats have all amino acids. Plants sometimes have only a few amino acids, though Soy has all of the amino acids. Soy is the essential meat/dairy substitute for vegatarians and people with lactose intolerance..
Meats (Lower fat fish, chicken, eggs are great), nuts, beans (soy/tofu has been found to grow better hair), whole grains (wheat and wheat germ), seeds, gelatin, yogurt, all are great foods that contain protein. Of the proteins, wheat germ, soy, and gelatin are found as the source in many hair growing products.
Vitamins
Vitamin Bs are the main vitamin hair needs to improve. Vitamin B complex - Regulates the secretion of oil, keeps hair healthy & moisturized. Of the Bs Biotin (B-7) and B-6 (Pyridoxine and Pyridoxamine) is most important, though another 3 Bs (B-5 (Pantothenic Acid), B-12 (Cyanocobalamin), and B-9 (Folic Acid)) are needed to work with Biotin.
2.) One should take a daily Multi-Vitamin for overall nutrition. In addition, take a B-complex to add additional B vitamins to help the hair.
Vitamin B comes from a number of natural sources, including potatoes, bananas, cereal, lentils, liver, turkey, and tuna. Brewer's yeast is an especially good source of Vitamin B.
Vitamin A is needed for healthy scalp. Vitamin C, Vitamin E (repairs skin and damage) and a few minerals such as iron and sulphur also help hair, which taking a multi-vitamin daily should cover.
Vitamin A is found naturally in many foods. dark green, orange & yellow fruits & vegetables contain Vitamin A. Sweet potatoes (yams), Carrots, Pumpkin, Spinach, Sweet peppers, Winter squash, Apricots, Cantaloupe melon, Mango, Liver (beef, pork, chicken, or turkey), Eggs
Growth Blockers
Stress, lack of sleep or rest, chemical stimulants (caffeine, smoking, carbonated soda stunt hair growth), heat/hot water weakens hair scales, too much sugar and fats are all bad on hair.
3.) Relax, sleep, rest, dont smoke or take caffeine, use cold to lukewarm water on hair if possible, avoid hair dryers, limit sweets and high fats.
4.) Massage scalp at least once a week to stimulate hair folicles.
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