I've always liked reverse polish notation.
I learned about it in high school and became addicted in college. My trusty HP48 calculator helped me through four years of engineering classes but left me unable to use a regular calculator! Seriously.
If you think about it, "regular" calculators are very counterintuitive. Sometimes they operate infix (such as "2 + 3 =" to get 5) and other times RPN (such as "4 sqrt" to get 2) and still other times in some sort of funky prefix (such as "2 + 3 = + 4" to get 9). And don't get me started on the memory functions (MR, MC, etc.). I'm surprised anybody can use a regular calculator.
Which brings me to the point of this post: I can't use the built-in calculators that come with Windows, Ubuntu, Mac, etc. I used to emulate the HP48 using Emu48, but the performance wasn't that great. Last week, I found grpn. Simple, fast, and beautiful RPN elegance. All I need now is a Windows version...