A while ago I bought an HP-16C calculator on eBay. It wasn't cheap and there was no manual; the calculator itself works fine and is in almost mint condition. Since then I've fallen in love with the device.
You probably think I'm nuts to be using a calculator that was discontinued in 1989 and only 203 bytes of memory. And I had to pay extra to get a PDF version of the scanned original manual.
Perhaps I am crazy, but here's why I love this little machine:
1. RPN. You either love this or hate it. This is my first RPN calculator and for me RPN is the right way to use a calculator. I read a short introduction to RPN tricks (of which there are very few, but filling the stack for repeated operations is one and using LST x to prevent the stack from moving is another).
2. The industrial design of HP calculators is pure art. They are the right size for your hand, the keyboard is clearly marked, keys are spaced far apart (which avoids fat fingers like mine) and the keys give good feedback on being pressed. And the calculator is slightly slanted so that when it's on the desk it's easy to type on.
3. Floating point with fixed display of decimal places. Just right for balancing your check book.
4. Hex/Dec/Oct/Bin modes plus the nice 'show' feature which can display a number in one of the other bases for a few seconds without changing base. Very handy when debugging.
5. And my favorite thing... the HP 16C is 128 mm wide and 79 mm deep. Notice anything interesting? 128 ENTER 79 / is... 1.62. Or the Golden Ratio. No wonder I love that thing so much.