Make magazine just put up instructions for how to assemble a cheap thermal regulator. This DIY regulator costs less than $50, which is way better than the $130 I spent buying a premade one. Thermal regulators (or controllers) are electronic devices which turn on or off an attached device based on the temperature of a system. I use a thermal regulator attached to a hair-dyer to keep a microscope incubator chamber at 37C. Other people use regulators to cool beer fermenter’s, cook food (sous vide), and heat laboratory samples.
DIY thermal regulator