DIY Test Equipment for Audio and Ham Radio Enthusiasts:
Heat Sink Tester
Ever wonder how to determine the thermal impedance of that bargain heat-sink?  It's pretty simple -- Mount a 100 ohm, 75 watt resistor to the base of the heat sink, applying thermal grease to reduce the impedance.   While I show an LM35CZ hooked up to a multimeter, you can use a thermometer from Radio Shack or Williams Sonoma.  The LM35 outputs 10mV per degree change in temperature.

This is a calorimetric method used in chemistry and physics laboratories -- albeit in circumstances where great precision is required the device under test (DUT) would be placed in a thermos bottle and surrounded by a conductive fluid with known thermal transfer characteristics. Until the advent of digital signal processing, thermometric methods were the best technique of measuring true "mean" voltages.

1) Measure the ambient temperature with the thermometer, or voltage from the LM35.

2) Apply sufficient voltage to the resistor to generate power of 10 watts in the resistor.  For the setup below, E = SQRT (P * R) =  SQRT (10 *100) = 31.6V

3)  When the temperature has stabilized, record it.  Subtract the ambient temperature from this temperature.

4) The thermal impedance of the heat sink is thus change in temperature for a given power dissipation. 

It is interesting to see just how much you can increase the heat sink's ability to wick away heat from the DUT by applying a bit of air movement.  Hook up a spare fan from a PC power supply.  If you have a variable power supply you can adjust the speed of the air flow.  Just a little bit of air movement makes a huge improvement in the efficiency of the heat sink.
You can get a first cut estimate of the heat sink's thermal impedance using Aavid-Thermalloy's "Thermal Resistance Tool"