I recently bought a DE0-nano FPGA development board, which I’m currently using to mine Bitcoins. It’s kind of a neat board, but one downside to it is that it uses linear regulators to provide the 1.2V core supply to the FPGA, and they’re incredibly inefficient at this. Only about 25% of the power supplied to them actually goes into the 1.2V supply; the other 75% is wasted as heat in the regulators. Since Bitcoin mining is very power-hungry, an awful lot of the power usage is in the 1.2V core supply and the regulators get really hot.
Fortunately, it’s possible to mod the board to use a more efficient external buck regulator to supply the 1.2 volts required by the FPGA core logic. (In theory this should even allow USB-powered mining; in practice this doesn’t seem to work for some reason.)