Unity3D Client And C# Server Framework
You can make it - Open Source MMORPG Platform
Real Time Strategy game engine supporting early Westwood classics
Vivid3D is a C++ IDE/3D engine, with python scripting.
PyCAPGE - Python Classic Adventure Point and Click Game Engine
A 2D game engine and level editor
Build Cross-platform Applications
Vivid Engine 2 is an open source 3D Engine coded using C# and C++.
IRB is a game creation toolkit
2D game engine for traditional platform games
Open Game Bootstrap
Single-/multi-player card game Mau Mau (similar to UNO®)
An extensible development framework for roleplay games.
Powerful html5 & javascript library for creating applications & games.
AMOS BASIC reimplementation in Java
A cross-platform Java 2D graphics library using OpenGL.