You can make it - Open Source MMORPG Platform
Updated emulator of Forgotten Realms: Unlimited Adventures
A real-time game development platform
Free and Open Game Programming Language for Windows and GNU/Linux
Gaming library
An advanced flexible game engine for Java.
Modular Gaming Open Source Persistent Browser Based Game Framework
Small OpenGL based game enegine
A 3D, fully concurrent game engine
Hungry Hero is an open source Flash game built on Starling Framework.
An extensible development framework for roleplay games.