That depends on how you define "viable". And "generate".

Peltier devices generate a voltage from a heat differential passing through a bi-metalic matrix. It's not a huge voltage, so the definition of "viable" comes in there, but it can be used to power low power things and works well for heater accessories. I first saw its use for wood stove fans that get powered just by sitting on the stove. I've also seen them power USB chargers for pellet stoves.

And then there's batteries that generate a voltage from submerging two types of metal in acid. And more modern battery designs might be doing it a bit differently but still no spinning magnets and coils. Obviously they are viable for powering many things, but usually themselves are powered from another source rather than using fresh acid for each charge, so the "generate" bit comes into question.

I think there's some others. Like fiction can be used to generate a static voltage and I'm pretty sure I've seen some tesla coils that use friction to generate their voltage. If you continuously generate that voltage, you could make a circuit out of it rather than shock high school kids or make their hair stand up, though I don't know what kind of amperage you could generate like that (that 5 figure voltage isn't fatal because of a lack of amps).

I asked an AI out of curiosity and, while I won't paste the response (feel free to ask one yourself), it gave a list of 20 methods, though I'd say this thread on its own covers about 9 of them, since some are different specific ways of doing similar ones (eg there were 4 based on moving something relative to a magnetic field).

hydro

Quickly boiling weird water then