Cook's Illustrated has come up with a way around this problem. They heat the stuffing in the microwave on HIGH for 6-8 minutes, stirring twice during cooking, until it reaches a temperature of 130 degrees. (If you don't have a food thermometer [get one!!] the stuffing should be too hot to handle with your bare hands.) Then stuff the turkey and place it immediately in the oven. NEVER stuff the turkey ahead of time; you are just asking for food poisoning.
Stuffing should be fairly dry when it is made. The bread, which forms the bulk of the stuffing, should be dried in a low oven. Bake it at 250 degrees F for 30-45 minutes until it's dry to the touch. If the bread isn't dry, the stuffing will become soggy and dense. The inside of a turkey is very moist, and as the turkey cooks it releases juice into the cavity, which is absorbed by the bread. I also like to cook any vegetables that go into the stuffing, especially harder root vegetables like carrots and onions. The heat inside a turkey cavity doesn't really get hot enough to cook those ingredients until they're tender.
Stuffing also expands as it cooks, so don't pack the stuffing into the turkey. I stuff the main cavity and the neck cavity, filling the cavities full but NOT packing.
Try making stuffing in the crockpot this year. The moist heat simulates the inside of a turkey, and the heat is so even it cooks the stuffing to perfection. By the way, if the stuffing mixture is baked outside the turkey, it's called 'dressing'!
There are so many different types of stuffing it would be impossible to list them all here. My favorite uses apples, onions, and raisins with oatmeal bread: My Great-Grandmother's Stuffing. Stuffings can include different types of bread, meats, seasonings, vegetables and fruits. Experiment and find your favorite recipe!