Launchers & Store Clients (Game Decompilations & Reimplementations): open-source projects that exist because someone reverse engineered a closed product, sorted by GitHub stars.
Open-source game library manager integrating Steam, Epic, GOG, EA, Ubisoft and more store libraries
C# Steam client application built on SteamKit2 for multi-account card farming and automation
Open-source launcher for Epic, GOG and Amazon Games libraries on Linux, Windows and macOS
Open-source Minecraft launcher managing multiple instances and Microsoft account login
Hello Minecraft! Launcher, a cross-platform third-party Minecraft launcher
Plugin loader for the Steam Deck that injects into the Steam client UI
Open-source CLI replacement for the Epic Games Launcher built on Epic's undocumented APIs
Custom Minecraft launcher for managing multiple isolated installations
Framework for Steam client themes and plugins via injection into the CEF UI
Desktop implementation of Steam's mobile authenticator (Steam Guard) app
Steam depot downloader built on the SteamKit2 reverse-engineered Steam protocol library
Custom third-party launcher for Final Fantasy XIV that reimplements the login flow
Alternative bootstrapper for the Roblox client with extra configuration
Open-source PlayStation 4/5 Remote Play client, successor of Chiaki
Unified game library for Steam, GOG and Humble Bundle (archived)
Syncs games from Epic, GOG, Origin, Amazon and other launchers into the Steam library
Lightweight unofficial Steam client for Android
Runs the Android build of Minecraft Bedrock natively on Linux and macOS
Simple unofficial GOG client for Linux
Linux CLI for Steam Guard 2FA codes and trade/market confirmations
Installs and switches between multiple Minecraft Bedrock (Windows 10) versions
Rust TUI Steam client driving steamcmd
Qt GUI alternative to the Epic Games Launcher, built on Legendary
Unofficial GOG.com downloader for Linux using the same API as GOG Galaxy
Open-source launcher/manager for running Roblox Studio on Linux
Utility that applies custom skins to the modern Steam client
Downloads a Humble Bundle library via Humble's internal API
Python tool to download a full GOG.com game and bonus collection for offline use
Unofficial launcher for Need for Speed: World community servers
Command-line tool to control Steam: download depots, workshop items, manage apps and accounts
Open source implementation of the GOG Galaxy communication service
Unofficial Amazon Games client (CLI) reimplementing the Amazon Games app's protocol
Online multiplayer client for classic Command & Conquer games on the CnCNet service
Launcher for Project Slippi, the rollback netplay and replay system for Super Smash Bros. Melee
Reverse engineered headers for Valve's Steam client interfaces
Open-source runtime to run the Roblox client natively on Linux
Free open-source replacement for the EA Desktop launcher for Linux and Windows
Unofficial Xbox Cloud Gaming app for Nvidia Shield TV
Library implementing the Xbox Cloud Gaming / home streaming WebRTC protocol
GOG download/install module for Heroic using GOG Galaxy's content-system APIs
Unofficial GOG client for Linux
Parser for Steam client appinfo.vdf and packageinfo.vdf files
Xbox Cloud Gaming streaming client for Nintendo Switch homebrew
Reimplementation of the Red Alert 3 game launcher
Unofficial Steam mobile client
Unofficial Epic Games Launcher client in JavaScript (login, friends, party, chat)
Simple CLI client for installing and maintaining Linux GOG games
Client for the unofficial Steam storefront and Big Picture web API