Mastercook contains the USDA nutrition database, along with additional ingredient nutrition data. So when you enter a recipe into it, it can figure the calories etc for you. If an ingredient isn't in Mastercook, such as a specific sauce or other product you include, you can enter it into the database from the label on your product and it will store it for you, then factor it into your recipe.
I love it
Unfortunately, it won't work on Vista so I'm trying to search out another product and haven't had any luck finding anything that I like as much as Mastercook.