PC Game Services (GameSpy, Battle.net, EA) (Online Service Revivals): open-source projects that exist because someone reverse engineered a closed product, sorted by GitHub stars.
Launcher adding community-hosted servers to Star Wars Battlefront II (2017)
Local server (ReCap) to play Darkspore after EA shut its servers
Revival server for Lucasfilm's Habitat online world (Commodore 64)
Client for Forged Alliance Forever, the community online service for Supreme Commander
Restores Halo 2 PC (Vista) online multiplayer after Games for Windows Live shutdown
Server reviving the Windows 7 and XP Internet Games
Custom server framework that brings back dedicated servers for Titanfall 1
Server for NieR Reincarnation
Emulator for early Steam servers
Server emulator for Need for Speed: Underground online play
Unofficial GGPO matchmaking server reimplementation
Custom server for LittleBigPlanet games
Private server for Love Live! School Idol Festival (SIF1)
Mass Effect 3 private server emulator
Python client for Forged Alliance Forever, the Supreme Commander online service
Reimplementation of Epic's master server for Unreal Tournament pre-alpha
SatellaWave, a tool to recreate Satellaview BS-X broadcasts for the Super Famicom
Dauntless revival that rebuilds its backend and multiplayer services
Lobby server of Forged Alliance Forever, which replaced GPGnet for Supreme Commander: FA
Server for Monster Hunter Frontier Z
Engine3 framework used by the SWGEmu Core3 Star Wars Galaxies server
Community launcher for Knockout City private servers after the official shutdown
Patches LittleBigPlanet games to connect to custom servers
Private server for Assassin's Creed Brotherhood multiplayer
Patcher DLL that makes Games for Windows - LIVE work under Wine on Linux and macOS
Dashboard to manage a Darkflame Universe LEGO Universe server
GameSpy replacement master server for the Project Reality: BF2 mod
Effort to salvage and preserve Lemnis Gate after its servers shut down
Recreation of the Fuser web services API after Harmonix shut it down
Private server for Deepworld
Revival project for the Pokemon Dream World Game Sync service
Resource tracking web app for Star Wars Galaxies emulator servers
Custom launcher for Marvel Heroes on MHServerEmu
Patches LittleBigPlanet games to use custom server URLs
Utility to manage Games for Windows - LIVE installs and content after the service wound down
Server replacement for Love Live! School Idol Festival After School ACTIVITY Next Stage
REST API of the Forged Alliance Forever community service
Revival server for the Pokemon Dream World / Global Link (Gen V)
Server reimplementation for Metal Slug Attack
Custom launcher for Monster Hunter Frontier private servers
Patches that add CnCNet online play to Command & Conquer: Yuri's Revenge
Server for Zynga's CityVille written in C#
Experimental online backend for Driver: San Francisco
Reverse-engineering research on the Games for Windows Live xlive.dll
Open-source game server for the 2014 Star Wars: Clone Wars Adventures client
Server emulation research for SkySaga: Infinite Isles
Web services that replace GameSpy online play for C&C Generals: Zero Hour
Classic Asheron's Call emulator based on the GDL project
Community server for the Drone Racing League Simulator after its servers shut down
Drop-in GameSpy master server replacement for SWAT 4
Substitute for GameSpy's master server for classic PC games
Emulators for Ubisoft's Game Service and Ubi.com network
Dummy dplayx.dll so old DirectPlay games run without DirectPlay installed
Metagame backend server for the Spectre Divide revival
Backend emulator for old games that use the Activision DemonWare services
Patches that move Unreal Tournament 3 online play to the OpenSpy GameSpy replacement
Patches and replacement server that bring back the discontinued game Girl Alone
Monster Hunter Frontier ZZ launcher that works without Internet Explorer
Restores dedicated server hosting in the cancelled Halo Online
Custom server for the defunct Big Bang Racing
Tunnel server for CnCNet, the revived online service for classic Westwood C&C games
Server for Minecraft Classic multiplayer after Mojang shut down the official one
FESL authorization and GameSpy servers for Battlefield 2142
CnCNet 4.0 client and server for online play of classic Command & Conquer games
Web services backend for the OpenSpy GameSpy replacement network
Launcher and installer for Soapbox Race World, the Need for Speed: World revival
Server for Tanki X
Launcher for Dragon's Dogma Online private servers
Cross-platform reimplementation of multi-version O2Jam servers
Server code for Age of Empires Online after Microsoft shut it down
Replacement GameSpy master server for Unreal-engine and other classic games
Server source of the Battlestar Galactica Online 2.0 Resurrection project
Community private server for the discontinued game Dreadnought
Spring Boot server for Forged Alliance Forever (Supreme Commander)
Reverse-engineered GameSpy SDK 2000-2005 emulator for Halo CE and Battlefield 1942/Vietnam
Server for Need for Speed: World after EA shut it down
CnCNet map server for classic Command & Conquer games, rewritten in Python
Sonic Runners custom server in C#
Reverse engineering and revival of the dead mobile game Zombie Cafe, with a Godot rewrite
Protocol documentation and local server for Angels Online after its shutdown
Galactic War backend server for Forged Alliance Forever
Astron-based private server for Pirates of the Caribbean Online
Mini emulator of the Games for Windows Live Marketplace server
Open-source LittleBigPlanet server, successor to Project Lighthouse
Local server for the Chinese version of Magia Record after its shutdown
Server emulation for U.B. Funkeys
Game server for Love Live! School Idol Festival 2
Ranked ladder backend for CnCNet Command & Conquer online play
CnCNet 5 tunneling service for online C&C games after Westwood Online closed
Private server for Mass Effect Andromeda multiplayer after EA servers shut down
Android revival of the abandoned mobile game Zombie Cafe
Account management portal for City of Heroes private servers
Patcher for Marvel Heroes client executables to use MHServerEmu
Restores Saints Row 2 PC multiplayer via OpenSpy after GameSpy's shutdown
Holmes server implementation for Harmonix Milo-engine games
Asheron's Call emulator written in LPC
Launcher for the Commonwealth Global Agenda private server
Proof-of-concept LEGO Universe server in C#
ICE P2P connection proxy for Supreme Commander: Forged Alliance on FAF
Reverse-engineered GameSpy emulator for Battlefield 2 and similar games
Launcher and manager for Marvel Heroes Omega with a local MHServerEmu
Local offline server for FIFA 20 Ultimate Team
Go backend for Battlefield Heroes with FESL TCP/UDP protocol
Account panel and management for City of Heroes servers
Reverse-engineered web revival of the multiplayer game Code Mafia
Wireshark dissector plugins for reverse-engineered GameSpy protocols
Replacement login server for Global Agenda
Reverse-engineering results and proof-of-concept server for Global Agenda
Client patches for Ratchet & Clank: Up Your Arsenal on the Horizon private server
Server emulation of Skyrama for preservation
C++/Qt launchpad for SWGEmu Star Wars Galaxies servers
Lightweight version of MHServerEmu for Marvel Heroes
ASI plugin that points Mass Effect 3 at Pocket Relay Blaze replacement servers
Fake Pokemon GTS server for Pokemon Black/White, based on IR-GTS-BW
Unity-based server sandbox for The Chronicles of Spellborn
Revival of the vf.net server for Virtua Fighter 4
OpenSpy web services that replace GameSpy HTTP/SOAP endpoints
Library to talk to Xbox LIVE and Games for Windows LIVE services
Battlefield 2 GameSpy login emulator
Connection and multi-client utility for Monster Hunter Frontier
Offline servers for older versions of Need for Speed: World
C structures and functions for the XBAND modem network protocol
GameSpy master server for C&C Renegade, run by CnCNet
Node.js master server for old Unreal Engine 1 games using GameSpy v1
GameSpy emulator that restores Section 8 and Section 8: Prejudice online play
Battlefield 2142 Stella stats web service for OpenSpy
Proxy DLL that redirects Battlefield 2142 to OpenSpy servers
Patch that moves Red Alert 3 (Steam) online play to the OpenSpy GameSpy replacement
ChatMonitor service for the revived GameSpy Arcade on OpenSpy