I generally divide Casio's programmable calculators into two groups: the older models with Defm/array memories, and the newer ones that use list variables. The Defm models may be older, but they offer some interesting advantages over the newer list-based models.
These programs enable easily and accurately calculating TVM and amortization problems on old Casio calculators in about 350 bytes of code.
The Casio fx-6300g graphing calculator has a subtle bug that affects using certain comparison operators against array memories, which will effectively cause the two sides of the comparison operator to be swapped. Here's how to trigger the bug, and what you can do to avoid it.
Despite being released about 4 years after the TI-82, the TI-86 does not include median-median regression, which can be a useful method of linear regression for data with extreme outliers. Fortunately it's a simple enough algorithm to implement in a small program.
Here's a small TI-86 program for twiddling Linux/UNIX permission bits and seeing both the symbolic and numeric results. The simple ones (750, 640, 755, etc.) are easy enough to do in one's head, but it can start to get a bit messy when setuid, setgid, and sticky get involved.
I bought an old Casio fx-6300g graphing calculator from ebay (it's so tiny!), and couldn't find many programs written for it. Who would imagine that the internet wouldn't be awash in programs for an obscure, low-end calculator from 27 years ago? So, I wrote a few: prime factors, binomial probability, and negative binomial probability.