This week in my fridge, a bag of milk  turned sour before I even opened it, but I did not throw it out. I also over-baked a batch of brownies without burning them, but I did not throw them out either. Here are the things I do to salvage a variety of foods that others might throw out:
- Sour milk can be used in baked goods that ask for buttermilk. I made a batch of buttermilk pancakes that my children were eager to eat for breakfast or take to school. The remainder of the sour milk I put in labelled containers in the freezer for another day.
- Over-baked brownies or other baked goods that are not burnt but dried out can be revived by placing 1-2 slices of fresh bread (heels work well) with them and then wrapping it all in a plastic bag for half a day. The bread will give its moisture to the baked goods. 
- Yogurt past the date will still be fine to eat even two weeks beyond "best before." If there is liquid on top, simply stir before serving.
- Dried out bread (heels included) can be fed to the birds, but you can also dry them out completely in a slow oven to make bread crumbs. Or cut dried out bread into cubes to make your own croutons or a topping for oven-baked casseroles. Another option is to place the bread in the microwave for a short time with a slice of cheese until it melts. Eat immediately. Dried bread is perfect for French toast because fresh bread tends to get too soggy.
- Wilted celery and the top leaves can be added to soups. If you want to consume the celery raw, immerse in ice water for about an hour, and it will become firm again.
- Small amounts of leftover cooked chicken can be cut finely. Add mayonnaise and relish to taste for a delicious sandwich topping.
- Bruised apples or pears can be peeled and sliced to remove the bruises. Then boil in a small amount of water until soft. Drain and add sugar and cinnamon for home-made chunky applesauce. If you prefer it smooth, put it in the blender.
- Bananas with brown marks on the skin might still be fine inside. If they are not, mash and make into banana muffins. About three mashed bananas make about one cup; they can be frozen for future use.
- Leftovers can be taken in lunches the next day. They can often be incorporated in the next day's dinner. For example, leftover cooked vegetables can be pureed and added to the next day's spaghetti sauce.
- Egg whites leftover from a recipe that calls for only egg yolks can be beaten until stiff and then added to cake or waffle batter. The end product will be lighter and higher.
- Egg yolks leftover from a recipe that calls for only egg whites can be stored in the fridge with a little water to cover. They can be added to a batch of scrambled eggs, fried rice, breads, cakes, cookies, or sauces. 
 If you live outside of Ontario, Canada, just imagine a one litre or one quart bottle or carton of milk.
 My mom's tip.
 The tips for extra eggs come from page 163 of the More with Less Cookbook by Doris Janzen Longacre, 1977.