This is from Cook's Thesaurus regarding Fat Substitutes:
For Baking:
Reducing fat will give baked goods a denser texture; to correct for this, try increasing the sugar in the recipe and/or beating the egg whites and folding them into the batter. Also try using a softer flour, like pastry or cake flour.
Applesauce can replace up to ¾ of the shortening in many recipes. Add with the liquid ingredients and reduce sugar in recipe if the applesauce is sweetened. OR
Pureed prunes can replace up to ¾ of the shortening in many recipes; it works especially well with chocolate. Add with the liquid ingredients. OR
Apple butter can replace up to ¾ of the shortening in many recipes, also reduce sugar in recipe if the apple butter is sweetened. Add with the liquid ingredients. OR
Fruit-based fat substitutes, especially good when baking with chocolate; add with the liquid ingredients. For best results, substitute only 3/4 of the fat with this.OR
Ricotta cheese works well in many yeast breads that call for solid fat. Substitute measure for measure. For best results, substitute no more than 3/4 of the fat with this. OR
Bananas, mashed, substitute measure for measure. OR
Omit or reduce in many recipes for quick breads, muffins, and cookies. you can reduce the amount of fat in the recipe by about a third without seriously compromising the quality.
Oil - Avoid substituting oils for solid fats when baking cookies, cakes, and pastries; it will make the dish greasy and dense. If you must do so, substitute 3 parts oil for every 4 parts solid fat and consider increasing the amount of sugar and eggs in the recipe. Pie crusts made with oil aren't as flaky as those made with solid fat.