An Atmos clock is a special kind of clock driven by changes in ambient temperature. It does not require rewinding or batteries. If it would not wear down after some hundred years, it could run forever.
The first such clock was constructed in 1928 by the French engineer Jean-Leon Reutter. The technology was later improved and patented by the Swiss watch and clock company Jaeger LeCoultre. Jaeger LeCoultre also makes the Reverso watch.
An Atmos works as follows: Inside a sealed capsule, a mixture of gas and liquid expands and contracts as the temperature rises and falls. This moves the capsule back and forth and so rewinds the mainspring by this movement. A temperature variation of one degree celsius is sufficient to power the clock for two days. The clock consumes so little power (2.5 * 10 ^-7 W) because the clockwork is nearly frictionless. This also greatly reduces the wear on the parts which results in a lifespan of probably more than 600 years.
Atmos clocks are terribly expensive because they are handmade. The prices start at $2600, but rare clocks can be worth up to $ 169'000. They do look very nice, too. The entire clock is under a glass dome, with the beautiful clockwork plainly visible.
Such a clock is probably the closest thing to a perpetuum mobile there is. (It isn't a true one because it relies on an external energy source.) In theory, such a clock could work for a very long time.