Flax seed comes in 2 varieties, brown and golden. The powder of both these varieties can be used as egg substitute. Seeds as well as the powder are available in health food stores, in the organic aisle of the regular grocery stores and sometimes in Indian grocery stores too. If buying flax seed/powder in bulk, it's advisable to store it in the freezer because it will become rancid.
Flax seed has a nutty flavor and so it works well in baked goods that are grainier and nuttier, such as waffles, pancakes, bran muffins, breads and oatmeal cookies.
The two measurements I have come across to substitute an egg:
1) Grind 1 tablespoon flax seeds to a powder in a blender. Add 3 tablespoons of water and blend until the mixture becomes viscous.
2) Blend 1 teaspoon flax seed powder with 1/4 cup water. (I have tried only this ratio so far)
For more information about flax seed as egg substitute check here
Check out recipes
using flax seed meal as an egg substitute.