Below is a list of a TON of substitutions if you happen to be missing a certain ingredient or maybe have a bunch of something you want to use in a recipe.  This list has come in handy a few times already for me!

ammonium bicarbonate - 3/4 teaspoon
1 tsp baking soda
baking powder (single-acting) - 1 teaspoon
1/4 teaspoon baking soda plus 1/2 teaspoon cream of tartar plus 1/4 teaspoon cornstarch
baking powder (double-acting) - 1 teaspoon
1/4 teaspoon baking soda plus 1/2 teaspoon cream of tartar plus 1/4 teaspoon cornstarch. Use 1 tsp for every 1 cup of flour.
baking soda - 1/2 teaspoon
2 teaspoons double-acting baking powder (replace the acidic liquid in recipe with non-acidic liquid)
baking soda - 1/2 teaspoon
1/2 teaspoon potassium bicarbonate
buttermilk - 1 cup (240 ml)
1 tablespoon lemon juice or vinegar plus enough milk to make 1 cup (240 ml) (let mixture stand 5-10 minutes)
cake flour - 1 cup (130 grams)
3/4 cup (105 grams) all purpose flour plus 2 tablespoons (30 grams) cornstarch
cake flour - 1/3 cup
1/3 cup all-purpose flour less 1/2 teaspoon
chocolate (bittersweet or semi-sweet) - 1 ounce (30 grams)
1/2 ounce (15 grams) unsweetened chocolate plus 1 tablespoon (15 grams) granulated sugar
chocolate (unsweetened) - 1 ounce (30 grams)
3 tablespoons (20 grams) natural cocoa powder (not Dutch-processed) plus 1 tablespoon (14 grams) unsalted butter, shortening, or vegetable oil
cocoa powder, Dutch-Processed - 3 tablespoons (20 grams)
1 ounce (30 grams) unsweetened chocolate plus 1/8 teaspoon baking soda. Also reduce fat in recipe by 1 tablespoon.
cocoa powder, natural unsweetened - 1 ounce (30 grams) unsweetened chocolate. Also reduce fat in recipe by 1 tablespoon.
coffee, strong - 1/4 cup (60 ml)
2 tablespoons (10 grams) instant coffee in 3 tablespoons hot water
corn syrup, dark - 1 cup (240 ml)
3/4 cup (180 ml) light corn syrup plus 1/4 cup (60 ml) light molasses
corn syrup, light - 1 cup (240 ml)
1 cup (200 grams) granulated white sugar (increase the liquid in the recipe by 1/4 cup or 60 ml)
cornstarch (for thickening) - 1 tablespoon (15 grams)
2 tablespoons (25 grams) all purpose flour
cream of tartar - 1/2 teaspoon
1/2 teaspoon white vinegar or lemon juice
cream - half-and-half - 1 cup (240 ml)
7/8 cup (210 ml) whole milk plus 2 tablespoons (25 grams) melted unsalted butter
cream, heavy (not for whipping) - 1 cup (240 ml)
2/3 cup (160 ml) whole milk plus 1/3 cup (75 grams) melted unsalted butter
flour, self-rising - 1 cup (140 grams)
1 cup (140 grams) all-purpose flour plus 1-1/2 teaspoons baking powder plus 1/4 teaspoon salt
flour, whole wheat - 1 cup (150 grams)
7/8 cup (120 grams) all-purpose flour plus 2 tablespoon (6 grams) wheat germ
honey - 1 cup (240 ml)
3/4 cup (180 ml) light or dark corn syrup plus 1/2 cup (100 grams) granulated sugar
lard - 1/2 cup (113 grams)
1/2 cup (113 grams) solid vegetable shortening
lard - 1/2 cup (113 grams)
1/2 cup (113 grams) plus 1 tablespoon (14 grams) unsalted butter
marhsmallow cream - 2.5 ounces
8 large marshmallows or 1 cup miniature marshmallows
milk (sweetened condensed) - 14 ounce can (396 grams)
blend 1 cup instant nonfat dry milk plus 2/3 cup (135 grams) granulated sugar plus 3 tablespoons (35 grams) melted unsalted butter plus 1/2 cup (120 ml) boiling water
milk (evaporated whole) - 1 cup (240 ml)
1 cup (240 ml) half & half
milk (whole) - 1 cup (240 ml)
1 cup (240 ml) skim milk plus 2 tablespoons (25 grams) melted butter or margarine
molasses - 1 cup (240 ml)
1 cup (240 ml) dark corn syrup
sour cream - 1 cup (225 grams or 8 ounces)
1 cup plain yogurt
sour cream - 1 cup (225 grams or 8 ounces)
1 tablespoon lemon juice or vinegar plus whole milk to fill 1 cup (240 ml)
tapioca, instant or quick-cooking - 1 tablespoon (12 grams)
1-1/2 tablespoons (20 grams) flour
vinegar - 1/4 cup (60 ml)
1/3 cup (80 ml) freshly squeezed lemon juice
yogurt, plain - 1 cup (225 g)
1 cup (225) sour cream