Whether for convenience or for reasons of dietary restriction, it's useful to know that you can substitute certain things for fresh egg whites in many baking recipes. Unlike whole eggs, egg whites are usually used for certain physical properties rather than their flavor. While flax seed meal makes a good substitute, it has a nutty flavor you should consider within the context of any recipe you are baking.
If you don't use many eggs, it can seem wasteful to keep an entire carton of fresh eggs on hand. Luckily, if you're in this situation but like to bake, both frozen and powdered egg whites are available. These store for much longer periods of time than fresh eggs; in the case of powdered eggs, sometimes several years. Also, unlike fresh eggs, you don't have to separate powdered or frozen egg whites before baking with them.
Many traditional baking recipes rely on dairy products, which can be disheartening if you're a vegan. It's extremely difficult to get vegan versions of recipes that rely heavily on egg whites such as meringues and angel food cake to come out as light and fluffy as you'd like. However, for other baking recipes that call for egg whites, substitute a 1:1 ratio of agar powder to water. Agar, a type of seaweed, can be found in health food stores and Asian supermarkets. Whipping this mixture yields a similar texture to egg whites, although it is not quite as fluffy. By repeatedly alternating between whipping and chilling it, you can achieve a somewhat fluffier texture.
Egg whites in baking are traditionally used both as binders and to give height and airiness. These substitutions perform admirably in terms of their binding properties, but do not achieve exactly the same airiness and texture as fresh egg whites. Even frozen and powdered egg whites do not yield exactly the same results as their fresh counterpart. If you are making a recipe with an egg white substitution for the first time, make a test batch before serving it to other people. That way, you can adjust the recipe as necessary -- or try a different egg white substitute, if you are dissatisfied with the first one.
If you dissolve 1 tbsp. of flax seed meal into 3 tbsp. of water and bring it to a low boil, you can create a fluffier substitution for either egg whites or whole eggs in baking recipes. However, be aware that flax seed in large quantities acts as a laxative. If your recipe calls for large numbers of egg whites, be careful how much flax seed meal you use as a substitute. Since it whips up fluffier than agar powder, use half agar and half flax seed substitutions for recipes that call for a large quantity of egg whites.