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:
In the 80s they brought out the HP Voyager series. Personally I think these are really cute. They're all the same hardware but they brought out different models targeting different use cases: 10C (basic calculator), 11C and 15C (scientific), 12C (financial), 16C (computer science)
The 16C I find particularly interesting - it was designed for programmers and had six buttons assigned A-F so you could type in hexadecimal numbers
The HP Voyagers aren't made any more, with one exception - the 12C was such a hit that the entire financial world standardized on it as their industry standard calculator. It's HP's best selling product of all time.
The modern versions look the same as the originals but internally they switched to an ARM CPU that's emulating the original CPU.
Remember the 41C? Well, when it was discontinued its replacement was the 42S, which seems to have built an even larger and more loyal following than its predecessor.
It doesn't look that impressive but this thing is cram packed with menus full of every feature imaginable. Probably the most impressive thing I saw was a program to solve the N Queens problem where the dot matrix display is used to graphically show the solutions.
If the 42S sounds intriguing you should check out Thomas Okken's Free42, which is not an emulator but rather a software recreation of an HP 42S. FLOSS and available on basically every operating system including Android/iOS:
On Debian systems you can `apt install free42-nologo`.
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