Nintendo Online (Online Service Revivals): open-source projects that exist because someone reverse engineered a closed product, sorted by GitHub stars.
Servers for Pokemon Gen IV/V online features such as the GTS
Server project that revives Monster Hunter Tri online play on Wii
Patcher that installs revived WiiConnect24 and Japanese Wii channels pointed at WiiLink
Revival of the 3DS Nintendo Zone hotspot service
macOS automation of a Nintendo 3DS StreetPass relay (HomePass)
Miiverse clone framework for 3DS and Wii U
Custom server that revives the shut-down Sonic Runners mobile game
Keeps the discontinued Wii U YouTube app working by patching it to a revived backend
Replacement server for the Nintendo 3DS eShop by Aftendo
Aroma module that redirects Wii U BOSS/SpotPass setup to Pretendo Network
Decoder and in-browser player for Flipnote Studio and Flipnote Studio 3D animations
Reimplemented Miiverse service API for Wii U and 3DS
Security fixes for RCE bugs in Wii U online multiplayer games, for use on revived servers
Generates News, Forecast and other Wii channel data files for the RiiConnect24 revival
GUI patcher that installs WiiLink's revived Wii channels and WFC
Web UI for Pretendo's Miiverse replacement, served to the Wii U and 3DS applets
Patches WiiWare titles to use Wiimmfi instead of the closed Nintendo WFC
Server that replicates the closed Flipnote Hatena for Flipnote Studio on DSi
Reimplemented NEX game server for Splatoon on Wii U
Reverse-engineering documentation for Flipnote Studio file formats and Hatena network
Tools to archive data from Nintendo Wii U and 3DS game servers before shutdown
On-console 3DS dumper of NEX RMC traffic, used to reverse engineer game server protocols
Reimplemented game servers for Pokemon X/Y/Omega Ruby/Alpha Sapphire online features
Implementation of the Game Boy Mobile Adapter GB protocol for custom servers
Revived backend for the Japanese Wii Demae (food delivery) Channel
Reimplemented Nintendo Network friends server for Wii U and 3DS
Reimplemented NEX game server for Mario Kart 7 on 3DS
Go server the Wii uses to send and receive WiiConnect24 mail
Large archive and browser of Miiverse posts saved before shutdown
Revived backend for the Wii no Ma (Wii Room) Channel
Go implementations of Nintendo NEX protocol definitions for replacement game servers
Docker deployment of the DWC Nintendo Wi-Fi Connection server emulator
Reimplemented NEX game server for Super Mario Maker on Wii U
Server that revives the Japanese Mobile Adapter GB online services
Fake Pokemon GTS server for DS games with AltWFC support
Runtime patcher that points Wii games at WiiLink WFC instead of dead Nintendo WFC
File generator that recreates Wii Forecast Channel data feeds
Replacement Wii U eShop UI
Revival of the Wii Shop Channel
Patches the Wii Shop Channel to connect to a custom shop server
mitmproxy setup to intercept and inspect Nintendo console traffic
Python parser for Flipnote Studio 3D's reverse-engineered .kwz format
Revival server for Animal Crossing: New Leaf online play
Arduino emulation of the Mobile Adapter GB for real Game Boy hardware
File generator for the revived Wii Nintendo Channel
On-console Wii U network traffic dumper for protocol reverse engineering
Kaitai Struct definitions for reverse-engineered Wii channel data files
Downloads Forecast and News Channel data so the channels work in Dolphin
Template for reviving the shut-down Nintendo Anime Channel on 3DS
Swapnote / Nintendo Letter Box clone for the 3DS web browser
Reverse-engineering notes for the 3DS Miiverse client
DLL hook that redirects Cemu's nintendo.net requests to Pretendo servers
Patches Wii IOS RSA key and mail checks so WiiConnect24 works with RiiConnect24
3DS plugin that keeps Pokemon Bank usable offline when its servers go down
Server that runs the revived Wii Shop Channel for Open Shop Channel
Reimplemented NEX game server for Minecraft: Wii U Edition
Patches the Wii Shop Channel to work with the Open Shop Channel
Replacement for Nintendo's SOAP services (eShop/title metadata) for Wii U and 3DS
Pretendo NEX game server for The Legend of Zelda: Tri Force Heroes on 3DS
Patches a Wii so its Message Board mail works with WiiLink Mail
Installer that sets up Pretendo Network services on a Wii U
SOAP communication templates for Wii Shop Channel revival servers
Research on Pokemon Dream World Game Sync protocol for Gen V DS games
Reimplemented BOSS/SpotPass content distribution server
Customizable gamertag for the Wii based on intercepted game launch data
Aroma plugin to choose between Pretendo or another network for Wii U online
Wii U patches that point the Miiverse applet at Pretendo's Juxtaposition
Parser/viewer for NEX connections in Wireshark captures
Original reverse-engineered Flipnote Hatena server
Generates patches that point Flipnote Studio 3D at custom servers
RiiConnect24 server scripts that run WiiConnect24 mail
Replacement for 3DS Ninja and Samurai (eShop/title) servers
Riivolution patch to play Mario Kart Wii online through Wiimmfi
Server for the Wii Message Board mail system (WiiConnect24)
File generator for the revived Wii News Channel
Open-source Flipnote Hatena server for Flipnote Studio on DSi
Web front end and admin panel for the DWC Nintendo Wi-Fi Connection server emulator
Demae Channel backend that orders food from a Wii through Deliveroo
SOAP server that handles Wii Shop Channel requests for the Open Shop Channel
Cross-platform proxy server that routes Wii U and 3DS traffic to Pretendo
Pretendo HPP server that revives the Swapdoodle messaging app on 3DS
Revival of Nintendo TVii for the Wii U
Generates and validates Wii Numbers for the revived WiiConnect24 mail
Pretendo NEX game server for Monster Hunter 4 Ultimate on 3DS
Installer for a Nintendo DWC server emulator with SSL for DS and Wii online play
Pretendo NEX game server for Pikmin 3 on Wii U
Ruby emulator of the Pokemon Global Trade Station server for DS games
Archiver service for Nintendo Network ID data before shutdown
Documentation of the Wii Shop Channel protocols for revival servers
Pretendo NEX game server for Yo-kai Watch Blasters on 3DS
Demae Channel backend that orders real food on the Wii through Just Eat
Flipnote Hatena replacement server for Flipnote Studio on DSi
Pretendo NEX game server for Steel Diver: Sub Wars on 3DS
RiiConnect24 server part that receives Everybody Votes Channel votes
Revived backend for the Wii Digicam Print Channel
HomePass utility that creates a 3DS StreetPass relay on OS X
Demae Channel backend that orders Domino's pizza from a Wii
Wii channel for Disconnect24, a WiiConnect24 mail replacement service
Custom server that revives the BBC iPlayer channel on the Wii
Revived server for the Wii Mii Contest Channel
Patches the Wii nwc24msg.cfg so Wii Mail works with RiiConnect24
Pretendo NEX game server for Super Smash Bros. for Nintendo 3DS
Pretendo NEX game server for Super Smash Bros. for Wii U
Script to create a Wi-Fi hotspot that Nintendo DS and Wii can use for WFC revivals
DeSmuME build adapted for online play through the AltWFC server
PHP utilities to compute Nintendo WFC, Wiimmfi and AltWFC friend codes and profile IDs
Pretendo NEX game server for Luigi's Mansion: Dark Moon on 3DS
Pretendo NEX secure server for Nintendo Badge Arcade on 3DS
Replacement for Nintendo's NAT check server used by Wii U and 3DS games
Replacement for SplatNet, the Splatoon 1 web stats service, on Pretendo
Pretendo NEX secure server for Super Mario 3D World on Wii U
Self-hosted community servers that bring back 3DS online multiplayer for several games
Pretendo NEX game server for Kid Icarus: Uprising on 3DS
Backend for WiiLink's revived Nintendo Channel on Wii
Pretendo NEX secure server for Pokemon Rumble World on 3DS
Revived WiiConnect24 server that checks whether a Wii friend owns a game
Fake Pokemon GTS server for Gen IV DS games, based on IR-GTS
Replacement for Nintendo's account.nintendo.net (NNAS) account server
File generator for the revived Wii Everybody Votes Channel
Pretendo replacement server for Dr. Mario: Miracle Cure on 3DS
Wii app to manage and analyze WiiConnect24 functions
Pokemon legality check server for Gen 6 3DS games on Pretendo Network
Pretendo NEX game server for Pokken Tournament on Wii U
Priiloader hack source that patches Wii games to use Wiimmfi
Pretendo NEX game server for Puyo Puyo Tetris on Wii U and 3DS
Pretendo NEX game server for Team Kirby Clash Deluxe on 3DS
Patches that let the Wii no Ma (Wii Room) Channel use revived servers
Utilities for the WiiConnect24 protocol and file formats
Revived backend for the Everybody Votes Channel
RiiConnect24 server part that serves Wii channel EULAs, region data and zip codes
Scripts that run a Nintendo 3DS StreetPass/SpotPass Homepass relay on a Raspberry Pi
Multiplayer match results server for Pretendo Network game servers
Pretendo secure game server for Mario vs. Donkey Kong: Tipping Stars on Wii U and 3DS
Work-in-progress Wii Shop Channel revival that serves the shop web pages
Pretendo replacement server for Dr. Luigi on Wii U
Pretendo NEX game server for Monster Hunter Generations on 3DS
Payload server for WiiLink WFC, the Nintendo Wi-Fi Connection replacement
Pretendo replacement server for RPG Maker Fes and RPG Maker Fes Player on 3DS
Pretendo NEX game server for Tetris Axis / Tetris Ultimate on 3DS
TV guide (EPG) API for the revived Japanese Wii TV no Tomo Channel
Utility to patch the Japanese Wii no Ma channel for WiiLink servers
Patches Wii Sports Resort to download Miis from the revived Check Mii Out Channel
WiiConnect24 standby client for Wii U mode (Cafe OS)
Pretendo NEX game server for Yo-kai Watch 2 on 3DS
Patches for the Wii Demae Channel so it can order real food via WiiLink
Pretendo NEX game server for Ice Station Z on 3DS
Pretendo NEX game server for Ironfall Invasion on 3DS
File generator for the revived Wii Check Mii Out Channel
Revived server for the Wii Check Mii Out / Mii Contest Channel
Pretendo NEX game server for Monster Hunter 3 Ultimate on Wii U and 3DS
Pretendo NEX game server for Monster Hunter 4 on 3DS
Pretendo NEX game server for Monster Hunter Stories on 3DS
Pretendo NEX game server for Monster Hunter XX on 3DS
Pretendo NEX game server for Puyo Puyo!! 20th Anniversary on 3DS
Pretendo NEX game server for Super Street Fighter IV 3D Edition on 3DS
WiiLink patches for the Wii Forecast, News and Everybody Votes channels
RiiConnect24 server that checks whether Wii friends own a game
Pretendo NEX game server for Bayonetta 2 on Wii U
Pretendo NEX game server for F1 2011 on 3DS
Pretendo NEX game server for Metroid Prime: Federation Force and Blast Ball on 3DS
Libraries that check input against Nintendo's NgWord filter lists, for Pretendo
Pretendo NEX game server for Resident Evil: Revelations on 3DS
Pretendo NEX game server for Resident Evil: The Mercenaries 3D
Pretendo NEX game server for Sonic & All-Stars Racing Transformed on Wii U
Pretendo NEX game server for Terraria on Wii U
Revived server for wus.wapp.wii.com, used by Wii channels to check friends' games