Brew Belt / Heat Belt
The Brew Belt is not thermostatically controlled, but you can control the heat by moving the belt up and down your fermenter.
To decrease heating, move the belt up the fermenter. To increase heating, move the belt towards the bottom of the fermenter.
You can also combine this with a standard timer plug to increase functionality.
If you're looking for greater control we suggest a heat tray or immersion heater.