HP has a K-12 Create-a-Calculator Contest.

I like that An Introduction to Reverse Polish Notation is linked on every page of this site. I've never liked the in-fix notation used by traditional calculators, requiring careful use of a "memory" register (the M, MC, M+ and M- buttons) and in-brain or on-paper management of operation order to do anything more complicated than dividing two numbers, or adding up a list. RPN calculators have advanced displays that include the contents of the last few levels of the "stack," but it's hard to imagine the equivalent features, namely multiple memory registers and an on-screen display of all memory contents, being as powerful or as easy to use. I hadn't thought calculators could get any more intuitive, even for basic operations like adding up numbers, until I first used an RPN calculator in high school.

Just thinking about RPN and my HP48GX makes me wish I still had opportunities to use it. Of all the computer-like devices I've owned and used, I've never felt such an aura of power and mystique as I do from my HP48GX. I wish everything used RPN, and I don't even know what that means.

GRPN is an RPN calculator for Linux, with a Gnome (X) interface.

VC is a command-line RPN calculator with special features for vectors of arbitrary dimension (including scalars), written entirely in Perl. I quite like this.