[comicjk]

Just to clarify why these aren't used everywhere: heat-to-power devices act as insulation (compared to just letting the heat escape). If you have something that you're trying to keep cool, like a CPU, a system that shunts heat straight to the surroundings will always give better cooling than a system that puts layers in between. Contrariwise, if you have a need for electricity, mechanical heat engines will almost always be more efficient. Solid-state heat-to-power only makes sense in a narrow set of cases which aren't suitable for direct cooling or heat engines.

[Matumio]

But there are some pretty cool applications. Apparently the heat difference between a buried water-pipe and the surrounding earth is already enough to power a wireless sensor, which can transmit data to localize leaks, for example.

[m463]

Voyager 2 and 1 are powered by thermocouples and a chunk of plutonium.

(I ordered them that way because Voyager 2 was launched first)