The HP-48 programming language, RPL, and its variants User RPL and System RPL are very dear to me. I spent hundreds, thousands, possibly billions of hours programming my two HP-48GX calculators back in high school.
RPL is unlike any programming language you have ever programmed in. There is somewhat of an introduction at the HP Museum, so I won't attempt to detail it. Basically, you have a stack, and your program looks like: