Other (Game Decompilations & Reimplementations): open-source projects that exist because someone reverse engineered a closed product, sorted by GitHub stars.
Reverse engineered Comanche Voxel Space terrain rendering algorithm
libdragon, an open-source Nintendo 64 SDK written without Nintendo's libultra
Open-source PSP SDK for homebrew development, written without Sony's SDK
Rust on the PSP with access to the system libraries
Automated tracker of Dota 2 game file and protobuf changes
OpenGL wrapper for PS Vita built on the reverse-engineered sceGxm GPU API
Automated tracker extracting information about each Roblox client version
Open-source PlayStation 2 SDK (ps2sdk and toolchain) written without Sony's SDK
EdB Prepare Carefully, a RimWorld mod
Photo!, a full custom replacement ROM for the Game Boy Camera
Technical reference for the original PlayStation hardware
Xbox One research wiki
Reverse Engineering Minesweeper: Reconstruct Minesweeper Source Code
Xbox 360 reverse engineering notes and utilities
Prussia, a Rust SDK for the PlayStation 2
Grit Game Engine
Custom ROM for the Game Boy Camera
Reverse engineering notes and tools for Zelda: Breath of the Wild
In-depth reverse engineering documentation of SEGA Chunithm
Reverse engineering a Neo Geo 161-in-1 cartridge to change ROMs
Automated tracker of Team Fortress 2 game file changes
Reimplementation of the Wii U nn_nfp amiibo library
Automated tracker of Deadlock game file and protobuf changes
Reverse engineering Pokemon Masters
Collection of reverse-engineering tools for the original Xbox
Rearrange the Nintendo 3DS HOME menu from a PC
Board-level reverse engineering of the original Xbox
Documentation of Mario Kart Wii internal structures
**OPEN SOURCE EPIC**
PSP Media Engine library exposing the ME core's native functions
Open-source replacement for the Xbox 360 XeCrypt suite
Little Big Adventure engine architecture documentation
Parts of the PlayStation 2 OSDSYS reverse engineered
Unofficial Rust bindings for the Nintendo SDK, used for Switch game modding
Tool to dump the ROM of the Xbox DVD Movie Playback Kit dongle
Tool for Dark Reign: The Future of War game data (osgameclones) (C#)
Unofficial technical documentation and code for the Sony PSP Media Engine
Reverse engineering of Virtua Cop 2 PC (1997)
Reverse engineering of Azrael's Tear (1996)
Reverse engineering the Creatures Evolution Engine (Creatures 2/3/Docking Station)
Reverse engineering of the 1988 game Carrier Command (DOS and other versions)
Reverse engineering documentation for 102 Dalmatians: Puppies to the Rescue
Reverse engineering of Legend of Kyrandia II: Hand of Fate
Reverse engineering project for the 1999 PS1 game PixyGarden
Reverse engineering project for Premier Manager 3
Reverse engineering Deuteros (Amiga, 1991) from preservation images
Verified reverse engineering of Dungeon Master II
Reverse engineering of Beneath a Steel Sky (DOS)