Unity3D Client And C# Server Framework
One framework for creating powerful cross-platform games
CTFs as you need them
Open Source MMORPG Framework
You can make it - Open Source MMORPG Platform
Arianne is an engine to develop multiplayer online games like Stendhal
JavaScript framework to build game engines, games, emulators and apps
Java Settlers
Mad Vandal's FortressCraft Mod Source Code
A synchronisation component for p5.js
2D game engine for traditional platform games
Single-/multi-player card game Mau Mau (similar to UNO®)
Java decentralized cross-platform network multiplayer game engine
Cross platform 2D/3D game engine written in C++11 & OpenGL ES 2/3
C++ game libraries plus server.
Open Source Virtual World and MMOG Development Platform
Cross-platform reimplementation of AMOS BASIC (runtime)
Fully fledged steam community SDK
Investigo - DX9 Performance and Debugging Toolkit