I have more than six different calculators. PC's have taken over most of their work, but I can't bear to get rid of them.
I received this calculator as a gift when I was taking an introductory programing class a long time ago. I never really used its advanced features, but it sure made binary - octal - decimal - hexadecimal conversions easy! However, only eight characters of a numeric value are show on the display, so you need to scroll to the left if you have a number greater than 255 represented in binary. Actually, I don't think that I ever used this for anything which could not be done on the Scientific View of the Microsoft Windows Calculator.
This calculator is now somewhat of a collector's item. It was made in the 1980's in the USA (probably in Corvallis, Oregon). The 16C was the only calculator that HP ever marketed solely for programmers. Some have sold recently on eBay for about $250.
This calculator is small but solid. It is 5 by 3 inches by slightly more than half an inch thick. The keys have a very nice positive response and very slight audible click. It is powered by 3 small watch/calculator sized batteries (A76), and has very good battery life. The 16C has continuous memory - Therefore data, chosen numeric formats, and any programs remain even when it is turned off.
This calculator uses RPN rather than conventional Algebraic logic. I think RPN is faster and easier (as long as you use a RPN calculator all the time).
This calculator is 'Keystroke Programmable', but only has 203 bytes of RAM (and this is shared between program memory and data registers). More information about the HP 16C and a look-alike Java Emulator is at http://www.hp16c.com/
The HP 16C calculator is a programmer's dream from years gone by.