Constant current or constant power loads are handy testing tools especially if you work with power supplies or want to do some extensive battery testing. Kerry D Wong has designed pretty handy load electronics which is capable to sing constant current and constant power as well.
In his design he used six IRFP150N MOSFET transistors to sink the current. Op Amps ensure that current is distributed evenly among all MOSFETs. Load parameters are controlled using Microchip MCP4921 DAC which comes with internal voltage reference. Arduino microcontroller can send commands to change voltage controlling load current. Arduino reads encoder to determine the DAC output. Kerry assembled his load on overkill heatsink. So he never had to worry about overheat.