Multi Theft Auto is a game engine
A lightweight online game framework
Arianne is an engine to develop multiplayer online games like Stendhal
The ultimate cataloguer
Full featured MUD server with all the trimmings.
Tugeny is a tournament organization software.
An unmodified version of the MMORPG Impressive Title by KovuLKD
A complete web framework to create online Fighting Fantasy adventures
Simulation tool for the casino game Roulette
JAVA websocket implementation for a bomberman-like game
A port of Sky3DS Diskwriter for Mac OS X and Linux
A comprehensive path-finding library for grid based games
Just another online poker software, but open source for once.
Multi emulator software for Windows
Game Communications Engine
Python Killboard Platform for EVE Online
Create your own online real-time browser-based MMO in your website
Earthlead is a free massive multiplayer online browser strategy game.