Blockmaster Engine was released for the HP 50g calculators in 2010, then updated in November 2012 by Marcos Navarro.

It is basically a program to create games (for the 50g only) in a graphic, easy and fast way. Just select your components from menus, with no programming needed. In other world, it is the same type of program as Game Maker, RPG Maker and other pre-made engine, which can be useful for those who like to design games, but not coding.

The only downside is that the instructions are only available in Spanish. Here is technical documentation, explaining how System RPL programmers can take advantage of the engine, along with documentation explaining how to create games with Blockmaster Builder. The author's site (also in Spanish) is available here.

Blockmaster Builder (to create games)
Blockmaster Engine (to run them)

Yah, HP is notorious for all types of calcs, all with rpn

Here's a translation of the introduction to the Block Master Builder:

First thoughts:
This tool lets you make games for the BME (v4.0) easily and quickly. You make your games by selecting components from a list and indicating how you want them to function

When you start using the Builder, the best thing you can do is try to get to know and experiment with the different types of components before thinking of tackling a full project. This manual will explain how components work and how to create them.

The next step is the creation of a Zone (ZxPack), which contains a group of components with an objective and either an exit or some other
way to win.

In order to create a Zone, we recommend that you start by making a small map containing the components the zone will have and some notes on how you want them to operate, as well as the objectives and how to win the game.

Using the Builder requires a certain precision, seeing as once created components cannot be removed from a Zone, though there are many characteristics which can be modified.

We call the person creating a Zone its "Master", who will have their own style and develop their own building techniques.

The Master can use standard graphics built into the Builder or create their own. On top of that, they can create and incorporate background graphics and animations to give a little style and make the game more appealing.

It's possible to organize a Zone's components into Groups, which lets you modify the state of several components at once (options in key G). This is important because some components such as the MBs {me: Mobile Blocks} and CEs {me again: Complementary Events} consume a lot of the CPUs power and it's better to only have the components presently being used activated and activate others as needed.

* I recommend also studying the Manual of Notes (BMA), which discusses more advanced themes. You can download it from the BM website. I've published the source code and some games which can serve as models as well.

There's loads of documentation and it's all really well organized. If anyone is curious about some part of the documentation just ask.

I don't know about the 39gII, but that hpcalc.org site lists it under "HP 49/50". The manuals (and the Google Code site) only mention the 50g. I don't know anything about the HP calcs, though, so i can't say.

I can help translate the documentation to English, but there really is a TON of stuff. Seriously, one of the pdf files is over a hundred pages long (though that includes a massive appendix with lots of charts and definitions)!

This would be awesome to port to Ti calculators

Indeed, and if it's possible on the 39gII too it might possibly have issues as well since it has the GUI around the screen that can't be erased in BASIC or whatever language it uses. That's unless the screen is large enough, though (I think it's larger than the 50g but I don't know by how much)

Also if ported to the 83+ it should probably just take advantage of the existing libs and use sprites for more customizing. It could be done in pure BASIC (requiring the user to copy data around, though, unless everything is external), hybrid (DCS libs), Grammer, Axe, ASM or C.