In the late 1970s, I owned a Hewlett-Packard HP-67 calculator. This was a brown coloured calculator, just a little bigger than a HP-45, but much heavier. It was programmable, and had a built-in card reader/writer. You could develop your software and then write it out to little cards. It was a funny brown colour, but it was really, really neat. I eventually sold it to someone in the 1980s.
I have been an avid HP calculator collector, and had a lot of them until a few years ago. Then I sold almost all of them on eBay. The calculators were all still working, but they were 30 or 40 years old. They were going to die. I wanted them to die and lose value while they were in someone else's collection, not mine. So I sold them. I still have one or two left, but they are ones that I currently use, they aren't collectibles.
One thing that has been really neat has been the rise of HP calculator emulators. I first found these on the Palm devices, and then saw them move to iPhones and iPads. I have a HP-11c (scientific) and HP-12c (financial) from RLM Tools. I bought these when they were on the Palm and now I have them for the iPhone. They are really good and I use them. My mind thinks in RPN with calculators.
That's not a real calculator, that's a HP-67 emulator on an iPad. It looks almost real. It's free. It comes with a couple of cards and they will load in and the action looks real. It's a full-featured scientific calculator, and it will do what the HP-67 did plus more. More registers, more this, more that. It's amazing. What it won't do is let you write your own programs. If you want that side of things, you should upgrade to premium and that costs $5. I did.
My original HP-67 in 1978 cost me about $600 AUS. Here is an emulation that works even better. It costs $5. Bargain. I bought it mostly for the nostalgia factor.