Once upon a time I wrote a post about whiskey stones, rocks that you cool down and then place into your drink to chill your Scotch without dilution, and why they are rather lousy at controlling your drink's temperature. The short version: Ice is so effective, per mass, at cooling your drink because its melting is a phase transition. Add heat to a mixture of ice and water, and the mixture sits there at zero degrees Celsius, sucking up energy (the "latent heat") as the solid ice is converted into liquid water. Conversely, a rock just gets warmer.
Now look at Joulies, designed to keep your hot beverage of choice at about 60 degrees Celsius. Note: I've never used these, so I don't know how well-made they are, but the science behind them is right. They're stainless steel and contain a material that happens to have a melting phase transition right at 60 C and a pretty large latent heat - more on that below. If you put them into coffee that's hotter than this, the coffee will transfer heat to the Joulies until their interior warms up to the transition, and then the temperature of the coffee+Joulies will sit fixed at 60 C as the filling partially melts. Then, if you leave the coffee sitting there and it loses heat to the environment through evaporation, conduction, convection, and radiation, the Joulies will transfer heat back to the coffee as their interior solidifies, again doing their level best to keep the (Joulies+coffee) at 60 C as long as there is a liquid/solid mixture within the Joulies. This is how you regulate the temperature of your beverage. (Note that we can estimate the total latent heat of the filling of the Joulies - you'd want it to be enough that cooling 375 ml of coffee from 100 C to 60 C would not completely melt the filling. At 4.18 J/g for the specific heat of water (close enough), the total latent heat of the Joulies filling should be more than 375 g \( \times \) 40 degrees C \( \times \) 4.18 J/g = 62700 J. )
Unsurprisingly, the same company offers a version filled with a different material, one that melts a bit below 0 C, for cooling your cold beverages. Basically they function like an ice cube, but with the melting liquid contained within a thin stainless steel shell so that it doesn't dilute your drink.
Random undergrad anecdote: As a senior in college I was part of an undergrad senior design team in a class where the theme was satellites and spacecraft. We designed a probe to land on Venus, and a big part of our design was a temperature-regulating reservoir of a material with a big latent heat of melting and a melting point at something like 100 C, to keep the interior of the probe comparatively cool for as long as possible. Clearly we should've been early investors in Joulies.