Base: Quaker Oatmeal Muffin mix (comes in a bag)
Follow instructions on bag, substituting canned pumpkin for water, and adding 2 tablespoons of Pumpkin Pie Spice. Be aware this recipe uses half of the bag. I recommend making the two batches separately instead of trying to make it all at once, the results tend to be better.
Note: I found that this produced slightly dry muffins, if you prefer them to be more moist, see the recipe for Pure Pumpkin Muffins.