Most ice cubes are cloudy due to a combination of air bubbles from dissolved gas, from impurities within the water.  Directional Freezing is a method to create crystal clear ice cubes.  The ice only freezes from one direction.  As an example, one way to achieve this is to insulate all the sides of your metal ice cube tray except for the very top of the tray.  The ice will freeze from the top down, creating a clear layer of ice on the top of the ice cube.