Essentially a temperature-activated fan, the Radiator Booster has been around for a few years now. Just recently, however, the new-and-improved MK3 model was introduced.
The Polypropylene device sits on top of your radiator, hooked up to mains power - it reportedly costs less than 50 cents a year to run. A red LED on the unit indicates that it is on, and waiting for the radiator to exceed a temperature of at least 30ºC (86ºF). Once that happens, the Booster's fan will kick in, and the light will turn green. After the radiator has dropped back below that "threshold temperature," the fan will once again stop, and the LED will turn to a flashing green.
Because the Radiator Booster puts a larger amount of heat out into the room where people can feel it, users should be able to set their thermostats lower, yet maintain the same level of comfort that they did before getting the device. According to a 2009 study conducted by the UK's Energy Saving Trust, Booster users were able to turn down their thermostats by an average of 1 to 3 degrees Celsius, resulting in energy cost savings of about GBP140 (US$220) per year.
A list of retailers in various countries can be found on the Radiator Booster website. The MK3 recently became available on Firebox, where it's selling for GBP24.99 (US$39).