From the Adafruit Forums: DIY Constant Current Dummy Load
Friend of Adafruit, George Graves, showed off his DIY Constant Current Dummy Load design on the forums. The design is based off of the EEVblog design shown on episode #102. The purpose of the device is for when you need a constant load to test things like power supplies, or batteries. No matter what the voltage coming out of the power supply, the constant current dummy load will adjust automatically to provide the same amount of current.