Retro Handhelds & Calculators (Firmware & Embedded): open-source projects that exist because someone reverse engineered a closed product, sorted by GitHub stars.
Custom firmware/OS overhaul for the Miyoo Mini and Mini+ handhelds
ArkOS custom Linux firmware for Rockchip-based handhelds (RG351, RGB10, etc.)
ROCKNIX immutable Linux distribution for ARM handheld gaming devices
Minimal custom launcher and libretro frontend for many retro handhelds
Third-party TI-84 Plus CE / TI-83 Premium CE emulator with developer features
Third-party open-source operating system for TI z80 calculators
Custom firmware for BittBoy, PocketGo, PowKiddy V90/Q90/Q20 and other Miyoo handhelds
Handheld firmware for Anbernic RG351/RG552/RG353 and compatible devices
Enhanced custom OS for the TrimUI Smart Pro handheld
JELOS Linux distribution for handheld gaming devices
Multi-platform emulator of TI-Nspire calculators
Custom firmware for Miyoo, TrimUI and Anbernic handhelds
Community C/C++ toolchain and libraries for the TI-84 Plus CE
Emulator collection firmware for the Nintendo Game & Watch
Code and research for hacking Tamagotchi Friends/Tamatown toys
Custom launcher for Miyoo Mini and Mini Plus handhelds
Reverse engineered schematics of the Game Boy DMG-CPU-B chip
Knulli custom Linux firmware for Anbernic, TrimUI and other handhelds
Panic Playdate RE notes: file formats, server API and USB commands
C++ libraries for Game Boy Advance link cable and wireless adapter protocols
Hardware-agnostic first-gen Tamagotchi emulation library (from ROM reverse engineering)
Information and tools for the SF2000 handheld console
Scripts and notes for working with the Nintendo Game & Watch hardware
Real Tamagotchi P1 emulator for Arduino UNO using the original ROM
MinUI custom launcher firmware for the Anbernic RG35XX
Cross-platform explorer and debugger for first-gen Tamagotchi ROMs
Retro-Go emulator firmware for Game & Watch with SD card mod support
Custom firmware patch for the Nintendo Game & Watch stock firmware
Cross-platform, CX-II compatible computer linking program for the TI-Nspire
TILP: transfer data between TI graphing calculators and a computer
Tamagotchi P1 emulator for the Flipper Zero
Tamagotchi P1 FPGA core for Analogue Pocket and MiSTer
RP2350-based recreation of the Pokewalker
Research into reverse engineering the Pokewalker from HeartGold/SoulSilver
Tamagotchi P1 emulator for microcontrollers built on TamaLIB
Emulator and debugger for Casio ClassWiz calculators
Tamagotchi development tools from firmware research
Decompilation and documentation of the Casio fx-991CN X CW calculator
Research notes on the Tamagotchi Paradise hardware and firmware
Pokewalker emulator
Work-in-progress Panic Playdate console emulator
Base project for writing custom Game & Watch firmware
CLI tool to flash and manage files on a Nintendo Game & Watch
Emulator and analysis toolkit for Nokia DCT3 phones
Tools used to reverse engineer the Game Boy Camera with an Arduino
Libraries (libticalcs, libticables, libtifiles) for TI calculator linking
Tool to flash the Game & Watch SPI flash via OpenOCD
Reverse engineered documentation of the Game Boy Advance LCD interface
Game & Watch core for Analogue Pocket and MiSTer driven by ROM dumps
Tools and scripts for running Linux on the HP Prime calculator
JavaScript library to talk to a Panic Playdate over USB serial via WebSerial
Technical specifications of Tamagotchi devices
H8/38606F emulator built for the Pokewalker
Reverse engineered schematics of the Game Boy DMG-CPU B chip
Human-readable matching decompilation of the Pokewalker firmware
Emulator of TI-89/92/V200 calculators for Linux and Windows
C++ library to read/write TI-z80 (82/83/84) calculator variable files
Native programming on the HP Prime G1 via reverse engineering
HP Prime calculator communication library, GUI and CLI
Third-party documentation of TI graphing calculator link protocols
Linux app to send and receive data from Casio fx-9860 calculators
Unix-style command-line programs to communicate with TI calculators
Reverse-engineered firmware of the Panasonic/Quasar/Olympia HHC hand-held computer
Decompilation of Casio ES PLUS GYX calculator firmware