I've yearned for an RPN calculator on the nspire so I can train myself in using it, so I created one in LUA.
In the default mode, it has 16 registers and has access to most common operations, and it also saves properly.
You can read more about what RPN is on Wikipedia. There's also a bit more about the app, as well as sample expressions in the readme.
If you want, you can copy and paste the script into other problems from both the calculator and the student software.
I'm pretty sure this is compatible with all nspires, including clickpads. If not, let me know.