Okay! So, I spent quite a bit of time trawling github and found a number of projects I haven't seen before, some new, some old. I've put together a preliminary list, but I haven't thoroughly vetted it, so feel free to exclude at your discretion. I'll split this into posts grouped by system. Many are WIP, and not all projects include a playable ROM, some are "compile it yourself" deals. Did my best to make sure none were already listed here, but I may have missed the odd one:
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SMS
------------Arcade Volleyballhttps://github.com/raphnet/arcadevolleyballArcade Volleyball is a simple volleyball game inspired by Pong. I remember playing it on a DOS PC, but this game was first made for the Commodore 64, and later for Amiga. This version was distributed on a floppy disk included with the Fall 1989 edition of the Compute!'s Amiga Resource magazine, as an executable accompanied with its source code.
This repository is a Sega Master System port based on the Amiga source code.
Arthur's Questhttps://github.com/haroldo-ok/arthurs-quest-smsA port of Piers Bishop's Game Boy game Arthur's Quest (GBDK-2020 + hUGEDriver) to the Sega Master System, built with SDCC and devkitSMS (SMSlib), following the sega-master-system-devkitsms skill.
Conway's Game of Lifehttps://github.com/jflores82/sms_gameoflifeConway's Game of Life - Sega Master System port.
This project is 100% human made, absolutely no AI of any kind was used for any part of the code, assets or documentation.
Game of Life (GoL) is a celullar automaton, zero-player game, developed by mathematian John Conway in 1970. The algorithm requires no proper input from the player, with the initial state determining how the game will evolve. Other than computation limits, there are no generational limit and there are no win conditions. It can achieve a stale condition where no evolution is possible.


Dragon Quest - Complete Quest Editionhttps://github.com/haroldo-ok/dragons-quest-smsYou have a "Dragon Quest II" on the list, but I couldn't find the link for it (and forum search isn't working), so not sure if this is a different project or not.
A complete, playable native Sega Master System port of Dragon Quest 1, built from the data files of the JavaDragonQuest1 fan remake using the devkitSMS toolchain from the sega-master-system-skill.
Nero Adventurehttps://github.com/flavio-rmc/sms-nero-adventureProtótipo de um jogo homebrew para Sega Master System, concebido como um prequel não oficial de Phantasy Star I. A campanha acompanha Nero antes dos acontecimentos do jogo original e acontece exclusivamente em versões originais dos labirintos já conhecidos de Algol.
Translated: Prototype of a homebrew game for Sega Master System, conceived as an unofficial prequel to Phantasy Star I. The campaign follows Nero before the events of the original game and takes place exclusively in original versions of Algol's already known mazes.
Pac-Man Master Museumhttps://github.com/lackoftrack27/Pac-Man-Master-Museumhttps://www.smspower.org/Homebrew/Pac-ManMasterMuseum-SMSAn accuracy-focused conversion of Pac-Man, Ms. Pac-Man, and Jr. Pac-Man for the Sega Master System.


Peng Ponghttps://github.com/haroldo-ok/Peng-Pong-SMSA port of the MSX beach volleyball game Peng Pong to the Sega Master System.
SMSHack: Dungeons of Doomhttps://github.com/haroldo-ok/gbhack-smsA Sega Master System port of GBHack — a NetHack-inspired roguelike originally written for the Game Boy Color.
Descend 15 procedurally generated dungeon levels, fight monsters, identify unknown potions and scrolls, keep a pet, rob (or pay) shopkeepers, and bring the Amulet of Yendor back to the surface.

Ultima IIIhttps://github.com/haroldo-ok/ultima-3-smsAn early playable SMS port based on RetroSven/ult3gbc, built with SDCC and devkitSMS
ZEPTON SMShttps://github.com/haroldo-ok/zepton-fmv-smsA Sega Master System port of REZ's PICO-8 voxel shmup ZEPTON (https://chiptune.itch.io/zepton), implemented as an FMV game: the voxel flyover is the original game's own renderer, recorded offline and streamed from ROM as simulation-indexed tile deltas (the smsvideo technique from haroldo-ok/sega-master-system-skill-for-claude).