The Micro Agnostic Gaming/Universal Storytelling engine is a barebones, single page game. This game is an active experiment of a bare essential tool kit to play any kind of TTRPG you wish, whether as starfaring psionic mercenaries, or the classic medieval fantasy of elves and dwarves.
The game is simple to play but open ended enough to be customized to your liking. In fact, it's so open ended it's released under CC BY 4.0 with minimal restrictions (more information in the document). Not only can you hack this easily into the game you want, but you can polish your hack up and sell it completely royalty free. Big thing is credit where it's due and not to use MAG/US in the title of your products.
Included with this document is the first companion book. It is also free because, while it's not exactly essential to have it to play, it does clear up the vagueness and word choice of the single page rules, as well as gives advice on running a game. This section is not released under creative commons, but you may freely adapt the advice in your own words.
Source: The Game Website.