I think most would agree that Hewlett-Packard made the best handheld calculators ever, epecially in the scientific market. The financial HP-12C, introduced in 1981, is still being manufactured today. Some financial companies still issue those calculators to their staff.
My personal history with HP goes back to my early childhood. Back in 1974-75, my cousin moved from Blekinge in southern Sweden to Stockholm after getting a job at Hewlett-Packard. For the first few months he lived with me and my family, until he was able to find an apartment. Through him, I got to see some early computers, once he brought home a computer, a plotter and a very early aucoustic modem. He connected it to a server back at HP and downloaded pictures to print on the plotter.
He also got my family our first calculator, an HP-21. It was the non-programmable version of the HP-25, with red LED display and rechargable battery pack.
In 8th grade we were allowed to start using calculators in math. So I got the then brand new HP-15C, a scientific programmable calculator that still is one of the best calculators ever made. I used it for a couple of years, and then switched to HP-28C and then (the next year) I upgraded to the HP-28S, with more memory. I also got the battery-operated infrared printer, HP82240A.
Finally, in 1990, I bought my last HP calculator, the HP-48SX. It was the replacement to HP-28S, and was later upgraded to become HP-48GX.
There was always a fairly large community dedicated to HP calculators. There were disks with programs (Joe Horn’s Goodies Disks) and all kind of information, including how you could open your calculator and solder on more memory… I also had several friends who used HP calculators, but the big majority of the students in my school did not understand the Reverse Polish Notation (RPN) used by HP on all their calculators except the financial models. That could be a good thing, as nobody ever asked to borrow my calculator when they forgot their own…
There are several emulators for HP calculators available, both for computers and for smartphones.
In Windows I use the wonderful Emu48 by Christoph Giesselink . It requires a copy of the ROM from a HP48SX or GX (depending on which calculator you want to emulate). The good news is that since 2000, Hewlett-Packard are generous enough to allow the use of ROM files even if you don’t own a calculator.
On my Android phone, until recently I used the Droid48 emulator . It is a HP48GX emulator, but it also has a HP-48SX mode with a simplified look. However, the other day I found a modified version of it called Droid48sx that looks just like the real thing.
There are also emulators for many other HP calculators, including HP-12C and HP-15C versions for Android which you have to pay for. There are free emulators for HP-18B/28C and HP-28S/42S, and many more.
Finally I want to share a picture of my HP-48SX, manufactured in the second week of 1990 (according to the serial number) and my 82240A printer (manufactured week 31 of 1987).