This is a low-level hack but nevertheless, aftermarket kits are available. The Mass Air Sensor in your car actually has 2 sensors in it, one for air flow (3 wires) and one for temparature (2 wires). The temp sensor is a thermistor whose resistance varies with temperature, in my case and probably most others, as the temperature goes up, the resistance goes down. The ECU takes this information and uses it to adjust the fuel/air mix as you drive.
Colder air is denser, and would require more fuel, which in turn will deliver more horsepower. If the temparature sensor is replaced with a $.30 resistor from Radio Schack in the right value, the engine will think the outside temp is about 0 deg F and adjust the mix accordingly.
Remove the sensor assembly from your car, determine which pins are connected to the temparature sensor, and measure the resistance. On mine, 65F corresponded with 2800 ohms. Put the unit with a thermometer in your fridge for a while, then check the resistance again and note the temp. These things react quickly so if you keep it under the burger, you'll get better numbers. Now calculate the ohms/degree curve. (T1-T2)/(R1-R2). This will probably work best if you convert the temperature to degrees Kelvin first to avoid divide by zero errors. Now calculate the resistance corresponding to say 0F and replace the thermistor in your sensor with such a resistor. Probably best to snip the wires to the sensor a couple inches back from the plug in case you need to go back to stock to pass local emissions requirements.
In my car, if the thermistor fails, the ECU reverts to an assumed temp of 37 deg C and puts an annoying message on the dash explaining that it's pining for the dealership again.