Ingredient Substitutions

I will go on record and strongly caution you against too much messing around with ingredients, especially leaveners, tenderizers, tougheners, etc--the ingredients that lend structure and texture to your baked goods.

Sometimes, though, you just have to find out for yourself.  Many substitutions will not adversely affect your finished product, especially substitutions of flavoring agents: 

My List of Substitute-able Ingredients
Milk + vinegar/lemon juice for buttermilk	Sour cream/creme fraiche	Butter for shortening and vice versa	Citrus juices
Liqueurs	Spices	Herbs	Citrus Zests
"Chunky" cookie mix-ins	Sugar (White for brown, etc)	Extracts and Emulsions	Fruits/juices for sorbets/granitas
Dairy (vary fat content)	Nuts	Fruits (fresh and dried)	Chips (chocolate, peanut butter, etc)

For an exhaustive database of ingredient substitutions, check out the site below. It is quite comprehensive.
      
Kansas State University's Ingredient Substitution Chart