When I make muffins, cakes, or brownies, I substitute skim milk for the milk, applesauce for the oil, and egg beaters for the eggs. If it calls for 1/2 c. of oil, use 1/2 c. of applesauce. Egg beater conversion is 1 egg equals 1/4 c. egg beaters. This really cuts down on the fat and it doesn't take away from the taste.
I don't recommend substituting applesauce for oil in cookies though. It makes them too doughy.