Pretty neat: Coffee Joulies

Coffee Joulies are miniature thermodynamic heat storage devices. They cool your coffee by absorbing excess thermal energy when it is too hot. This energy is stored inside the Joulies. When your coffee reaches the right temperature, the Joulies slowly release this stored energy, keeping your coffee at the right temperature.

From their kickstarter page:

This amazing feat of thermodynamics happens thanks to a special non-toxic material sealed within the polished stainless steel shell. This material is designed to melt at 140 degrees Fahrenheit, and absorbs a lot of energy as it melts. This is how Joulies cool your coffee down three times faster than normal. Once it reaches this temperature, the special material begins to solidify again, releasing the energy it stored when it melted. This is how Joulies keep your coffee warm twice as long.

The better insulated your cup, the better Coffee Joulies work. Simply using a lid goes a long way. This is because less heat is lost to the environment, and more is stored within the Joulies themselves.

That energy is, of course, the enthalpy of fusion. Think of an ice cube: the ice absorbs energy as it melts, maintaining the same temperature but cooling the beverage down. What these people have done is use a material that melts at a convenient temperature for hot beverages and that takes advantage of the reverse process that happens in the freezer — the transition back to a solid, releasing that energy, at the melting point.

The site doesn’t specify the material, but the various forms of paraffin wax have melting points around 140 ºF with large heat capacities and enthalpies. Which would be pretty much what you want to use.

(The bit about the lid and/or insulation keeping things warm works without the Joulies, too)