Stove Thermometer
Magnetic stove thermometer which attaches to your stove pipe to ensure the stove is operating at the correct temperature.
An essential addition to your toolbox, especially if you are unfamiliar with stove operation: if the stove is running too cold it can result in dangerous deposits forming in the chimney, and if it's too hot, your vents are open too far and you are wasting heat up the chimney.