Recently, I got the calculator app PCalc on my Mac, and its customisability is great, but by far the best feature is RPN. RPN (short for "Reverse Polish Notation") is a different way of expressing calculations, where instead of the operator going between the operands (like in "6 × 7"), it goes after them both (like in "6 7 ×"). While I definitely do not advocate that you write your math like this on paper, it's an amazing way to enter your math for a calculator.
With RPN, as you start entering a number, it will go into the x register. Then, you can press return, and it will [Push] the values in all registiers up one level. In this case, the value you typed into the x register will move into the y register. If you [Push] again, the value in the y register moves into the 1 register, and another would would move it to the 2 register, and so on. Physical RPN calculators would have a finite number of registers (perhaps four) and call them by different names (like x, y, z and t), but PCalc has no problem magically creating new reigisters as you [Push] more and more numbers into it, though rarely are very many used.
After pushing the value you entered from x to y, you can start typing again, filling up the x register with a new value. Then press + to add the values in x and y together. That's it! As you would imagine, pressing different operators will perform different operations, but that is not too difficult to master.
Once you get used to RPN, one powerful property you'll appreciate is that parentheses are never needed. Because you execute your operations one at a time, you can pick the order they happen in, without needing to use curvy lines to communicate with the computer. Even though I execute a quite complex calculation in the video: ⌊15 × 1.5⌋ + ⌊9 × 1.5⌋ + 10, the order of operations can be experessed simply by my choice to leave certain values "on the stack" in higher registers, so that when I perform operations on the x and y registers, they are left alone. See how from 0:05 to 0:09 in the video I'm able to calculate ⌊9 × 1.5⌋, and the multiplication only affects the 9, and not the 22 left in the 1 register from previous operations.
When entering numbers on a keyboard or touchscreen with RPN, I feel a similar sense of speed and control as I do when using Vim motions to edit code. I am very new to RPN, having only used it (at time of writing) yesterday evening and a little this morning, so I am excited to grow my proficiency with it over time.
Also, if you write words or code on a computer, and when I said "Vim motions" you didn't know what I meant, take a look at this video and give Vim motions a try. You're really missing out.