Butter or margarine are the most common products which can be used as a substitute for shortening in recipes in approximately equal amounts. Other products which can be used include vegetable oil and olive oil, but only if the shortening is to be melted, as for frying.
Lard, bacon fat or the drippings from a roast may all be used as a shortening substitute as well. Which substitute a cook chooses depends on how the shortening is to be used in a recipe. In baking cakes or other confections, it's best to use butter or stick margarine, but for biscuits or other breads, lard or bacon drippings may be substituted. Butter, lard, shortening and vegetable oil are all different forms of fat, and so may be used interchangeably when flavor profiles permit.