The HP 41C came out in 1979 and seems to have been one of the first calculators to build a real community around it, probably because it was designed to be expandable. Add-on software and peripherals were released for it.
People even "jailbroke" their calculators using what they called "synthetic programming". They figured out details of the internal instruction format and were able to do the equivalent of a VM escape exploit so they could do things HP hadn't intended:
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