Coffee Joulies are one of the coolest things I've seen in a long time. They absorb the heat from your too hot coffee and cool it down to the perfect drinking temperature. Then, they release all that extra heat into your coffee to keep it at that temperature for twice as long. It has to do with the melting and solidifying of the material inside the beans. It's all quite scientific, but you can read more about it on their website.
You can't buy Joulies yet, but they're working on getting them out in the market (these ones came from Kickstarter). Keep an eye on their website for your chance to get your hands on them.