I try to note substitutions within the recipes, but here is a list of some that I have found helpful... the intention here is not necessarily to provide alternatives for when you can't use something (as with allergies) but more to provide suggestions of ways to fill in when you don't have an ingredient, or to replace prepared foods with their scratch counterparts.
For allergies I recommend finding recipes that are designed to function without the specified ingredient, rather than trying to substitute things.
(this post will be updated as I find new things of course!)
(in alphabetical order)
Bisquick--1 cup--1 c flour, 1 1/2 tsp baking powder, 1/2 tsp salt, 1 Tbs shortening (leave out shortening if using as a component in another recipe)
Cornstarch (as a thickener)--use double the amount of plain white flour
Eggs--baking soda, baking powder, or applesauce (it depends on their function in the recipe, see this guide)
Self-rising flour--1 cup--1 c flour, 1/2 tsp salt, 1 tsp baking powder