Ever since i saw the the fanless CPU cooler with it's boiling coolent, I have been wanting to develop an off-the-shelf design. R123 freon (Dichlorotrifluoroethane), with a boiling tempature 27.82 °C (82.08 °F), would be ideal for a pumpless liquid cooling solution I believe. As long as you avoid exothermic reactivity in certain materials you don't want in your cooling system anyway. Because it is a liquid at room temperature, it is safe to handle. It has been used in commercial cooling systems for many years.