I've seen many coffee mugs that change their decoration depending on the temperature of its contents, but this has to be the bestest one in the history of bestest gimmicky coffee mugs. Evars. If you are a nerd like me, anyway.
Before it was only a Russian thing/concept by Art.Lebedev, but now it's available in the US for around £10, excluding shipping it across the pond. [Thinkgeek via Laughing Squid—Thanks Alyssa!]