What is a good butter substitute?

That depends on what you're making. If you're baking, I wouldn't risk a substitution. Baking is very precise, there is a delicate balance of ingredients in order to achieve the desired chemical processes and the right result. If you need a substitute for butter in a COOKING recipe, say for a sauce, or to pan fry vegetables, that's different. You could probably substitute margarine, Smart Balance, or a light and tasteless oil.