These are from the Better Homes and Gardens Cook Book - I use these all the time! Hope they are helpful to other people. These are just the most common ones, I'll keep adding to it.
*Baking powder, 1 tsp. - you can use 1/2 tsp. cream of tartar plus 1/4 tsp. baking soda.
*Bread crumbs (fine, dry), 1/4 cup - use 3/4 cup soft bread crumbs; 1/4 cup cracker crumbs; or 1/4 cup cornflake crumbs.
*Chocolate, semisweet, 1 ounce - use 3 tbsp. semisweet chocolate chips/pieces; or 1 ounce unsweetened chocolate plus 1 tbsp. granulated sugar.
*Chocolate, sweet, baking, 4 ounces - use 1/4 cup unsweetened cocoa powder plus 1/3 cup granulated sugar and 3 tbsp. vegetable shortening.
*Chocolate, unsweetened, 1 ounce - use 3 tbsp. unsweetened cocoa powder plus 1 tbsp. cooking oil or melted vegetable shortening.
*Cornstarch, 1 tbsp. (for thickening) - use 2 tbsp. all-purpose flour.
*Corn syrup, 1 cup - use 1 cup granulated sugar plus 1/4 cup water.
*Egg, 1 whole - use 2 egg whites; 2 egg yolks; or 1/4 cup frozen egg product, thawed.
*Flour, cake, 1 cup - use 1 cup minus 2 tbsp. all-purpose flour.
*Garlic, 1 clove - use 1/2 tsp. bottled minced garlic; or 1/8 tsp. garlic powder.
*Herb, snipped fresh, 1 tbsp. - use 1/2 to 1 tsp. dried herb, crushed.
*Honey, 1 cup - use 1 1/4 cups granulated sugar plus 1/4 cup water.
*Lemon juice, 1 tsp. - use 1/2 tsp. vinegar.
*Onion, chopped, 1 small (1/3 cup) - use 1 tsp. onion powder; or 1 tbsp. dried minced onion.
*Sour cream, dairy, 1 cup - use 1 cup plain yogurt.
*Sugar, granulated, 1 cup - 1 cup packed brown sugar; or 2 cups sifted powdered sugar.
*Tomato sauce, 2 cups - use 3/4 cup tomato paste plus 1 cup water.