Proprietary Enterprise Protocols (Network & Wire Protocols): open-source projects that exist because someone reverse engineered a closed product, sorted by GitHub stars.
Client for Fortinet's PPP+TLS SSL-VPN tunnels
Palo Alto GlobalProtect VPN client for Linux/macOS, based on OpenConnect
Pure JavaScript implementation of the TDS protocol for SQL Server
Python library for Windows Remote Management (WinRM)
Pure Go Oracle client implementing the TNS wire protocol
Pure Go Kerberos library for clients and services
POP/IMAP/SMTP/CalDAV/LDAP gateway to Exchange and Office 365 (EWS/WebDAV)
NFS client library
A Kerberos implementation built entirely in managed code.
node-steam plugin for the Dota 2 game coordinator
Dr.COM campus network authentication on HiWiFi routers running OpenWrt
Reimplementation of Sybase/Microsoft SQL Server TDS wire protocol
Command-line tool and library for Windows remote command execution in Go
node-steam plugin for the Counter-Strike: Global Offensive game coordinator
Pure-Python FinTS/HBCI banking protocol client
PowerShell Remoting Protocol for Python
Unofficial Dante Controller alternative for multiple platforms
Unofficial implementation of the Dante audio-over-IP protocol
Free home-banking app for Linux, Windows and macOS built on HBCI4Java
A cleaned-up and improved version of the jCIFS library
Python package for interacting with the Dota 2 game coordinator
NTLM/Negotiate authentication over HTTP
Unofficial H3C 802.1X campus network authentication client
Java FinTS/HBCI implementation with chipTAN, pushTAN and SEPA support
Open implementation of Microsoft Exchange MAPI/RPC protocols
PHP library to communicate with FinTS/HBCI bank servers
Linux kernel EoIP tunnel compatible with MikroTik
Open source Exchange ActiveSync implementation for syncing mobile devices
Pure Python implementation of the Tsinghua TUNet 2018 campus network auth protocol
Cross-platform third-party client for Guangdong China Telecom ESurfing campus auth (C)
Java library for interacting directly with Valve's Steam servers, based on SteamKit
JavaScript FinTS/HBCI client
Third-party Pidgin plugin for the Microsoft Lync/OCS protocol
Clean-room implementation of the Dante audio protocol
Documentation of the reverse engineered Ubiquiti UniFi inform protocol
Wireshark dissectors for proprietary SAP NI, Diag, RFC, Router, Enqueue protocols
Lightweight Steam client framework for Node.js on the Steam CM protocol
Pure Rust SMB2/3 client
Clean-room open implementation of the Unified Bus protocol
Command-line client for Check Point SSL VPN
Node.js module for interacting with the Steam store's undocumented web endpoints
Python SPNEGO authentication library
.NET client library for HBCI, FinTS and EBICS banking protocols
Node.js module for talking to the Team Fortress 2 game coordinator
Reverse engineered docs for the protocol between UniFi APs and controller
Node.js SteamClient implementing Valve's reverse-engineered Steam network protocol
Java port of SteamKit for Steam network protocol access
GSSAPI implementation of Microsoft NTLM (MS-NLMP)
Steam client for game servers on the Steam CM protocol
Node.js implementation of Steam's session crypto
Reverse-engineered Outlook drag-and-drop OLE clipboard protocol