According to About.com, white vinegar or lemon juice can be used as a substitute for cream of tartar when making meringue. Another option is to completely omit the cream of tartar from the recipe.
Cream of tartar is used in meringue to stabilize the egg whites and prevent issues such as weeping. White vinegar or lemon juice can be equally exchanged for cream of tartar in meringue recipes. Generally, either substitute works the same way as cream of tartar by stabilizing the egg whites. In certain recipes, a pinch of salt can also be used as a substitute for cream of tartar.