Baking powder, white wine vinegar and lemon juice are all acceptable substitutes for cream of tartar, depending on the recipe it is used in. Vinegar and lemon juice are good substitutes when stabilizing egg whites, and baking powder can be used in baked goods.
Cream of tartar is used in baked goods, meringues, frostings and sweet glazes. In baked goods, cream of tartar dissolves and becomes acidic, reacting with baking soda and causing baked goods to rise. Baking powder, available at any supermarket, contains cream of tartar and baking soda, and so makes a decent substitute.
In meringues, cream of tartar is used at a ratio of 1/8 teaspoon per egg. When substituting white wine vinegar or lemon juice, use at the same ratio. The two liquids are equally effective, but provide subtle flavor differences. Choose which to use based on the dish you're serving. For fruit-based meringues, such as a lemon meringue pie, lemon juice is the best choice, while vinegar may be more suitable in other whipped egg-white dishes. In frostings, cream of tartar prevents sugars from crystalizing, so lemon juice is an acceptable substitute.
Cream of tartar is the common name for potassium bitartrate, a white residue which forms on fermentation barrels used in making wine. This residue is collected and processed, forming the white powder sold in stores.