Messaging Protocols (Network & Wire Protocols): open-source projects that exist because someone reverse engineered a closed product, sorted by GitHub stars.
Socket-based TypeScript API for WhatsApp Web
Go library for the WhatsApp web multi-device API
Matrix-WhatsApp puppeting bridge
Self-hostable server compatible with classic AIM and ICQ clients (OSCAR protocol)
Facebook Messenger (MQTT) protocol plugin for libpurple
IRC gateway to other chat networks (OSCAR, MSN, Jabber, Twitter...)
Skype protocol plugin for Pidgin/libpurple (SkypeWeb)
Matrix bridge for Facebook Messenger and Instagram DMs
Discord protocol plugin for Pidgin/libpurple
WhatsApp plugin for Pidgin/libpurple built on whatsmeow
Qt FeiQ client for macOS implementing the FeiQ (IP Messenger extension) protocol
Unofficial Telegram MTProto server in Java
Third-party Microsoft Teams plugin for Pidgin/libpurple
Unofficial Telegram server in C# with channels, media, secret chats and calls
Unofficial Go library for Microsoft Teams internal APIs used by the official clients
Open-source Java TeamSpeak 3 client library using the full client protocol
Matrix-Google Chat puppeting bridge
Unofficial open-source Telegram server written in Go
Unofficial MTProto server in Rust that works with Telegram clients
Packet capture and analysis tool for reverse engineering the AOL protocol
Open Source reimplementation of Gadu-Gadu backend (official mirror of https://codeberg.org/or3e/poggadaj)
Zoom chat bots via reverse-engineered Zoom Web SDK websocket protocol
Multiplatform client for the MSN Messenger protocol