Sony & Sega Online (Online Service Revivals): open-source projects that exist because someone reverse engineered a closed product, sorted by GitHub stars.
Clean-room open-source custom server for LittleBigPlanet
Community DemonWare server to play Call of Duty: Modern Warfare 2 online on PS3 or RPCS3
Raspberry Pi bridge between a Dreamcast dial-up modem and the internet
Phantasy Star Online 2 private server project in C#
PS3 plugin that re-enables Steam connectivity in Portal 2 on PS3
Replacement for Sony's PS2 DNAS authentication servers
Restores user-generated content features for inFAMOUS games on PS3
Unified server suite for PlayStation Home, SVO, Medius and other PS2/PS3 online services
Open-source SCE-RT Medius server for PS2/PS3 online games such as Ratchet & Clank
Server implementation for Phantasy Star Online: Blue Burst
Medius server emulator for Ratchet: Deadlocked and other PS2 online games
Lobby server emulator for The Sims Bustin' Out (PS2) Online Weekend mode
Revival server for EverQuest Online Adventures (PS2)
Private game server for Mobile Suit Gundam: Federation vs. Zeon DX (Dreamcast/PS2)
Work-in-progress custom server for Gran Turismo 5 and 6 on PS3
Ship server for Phantasy Star Online, part of the Sylverant revival
Run-time patching disc for Phantasy Star Online and other online Dreamcast games
Metal Gear Online 1 private server for PS2
Phantasy Star Online: Blue Burst private server
Support library for the Sylverant Phantasy Star Online server
Tethealla server for Phantasy Star Online: Blue Burst
Login server for Phantasy Star Online on Dreamcast, GameCube and PC
Central shipgate server for Sylverant Phantasy Star Online ship servers
Shares a Raspberry Pi's connection with Dreamcast Broadband Adapter devices
Phantasy Star Online: Blue Burst server emulator built on Booma
DNAS server container for PS2 online revival
DNS proxy for reaching revived PS2 online game servers
Server emulator for Capcom's KDDI servers used by Japanese Dreamcast games
Drivers and servers to play Virtual-On: Oratorio Tangram (Dreamcast) online
Patches LittleBigPlanet PSP to connect to custom servers
C++ rewrite of the PSForever PlanetSide 1 server
Docker wrapper for PSP ad-hoc (aemu) online play servers
Small DNS server for private game servers such as Sylverant PSO
Python Medius implementation for Ratchet & Clank: Up Your Arsenal online play
dinput8 wrapper with fixes for Phantasy Star Universe on the Clementine server
PlanetSide 1 launcher with login for PSForever servers
Patch server for the Phantasy Star Online series
Client patches for Ratchet: Deadlocked PS2 players on the Horizon private server
Single-binary Go version of DNASrep, a PS2 DNAS replacement server
Script to get a Dreamcast online through a PC-DC server connection
Revival backend for The Last of Us: Factions (PS3) multiplayer under RPCS3
Revives online mode of Resident Evil Outbreak File #2 (PAL) on real PS2 hardware
Dedicated LAN server implementation for Warhawk on PS3
Pure Python recreation of the PS2 DNAS authentication servers for online games