Arianne is an engine to develop multiplayer online games like Stendhal
The ultimate cataloguer
Online War system for IL-2 Sturmovik and Iron Front in ArmA 2
The multiplayer game engine
Java Settlers
Playing Go with web-based turn-based Go server
scriptable game engine with built-in support for 2D graphics and sound
Asphyre is a cross-platform framework for Delphi and FreePascal.
plugin-based RPG manager in C++ and Qt5
Mad Science
Compact/ Chess Interchange Format
web cms for clans and esport related pages
The original Xivio.com World Server Core
An extensible development framework for roleplay games.
Modular Minecraft server plugin - commands, events, ...