Arianne is an engine to develop multiplayer online games like Stendhal
Java Settlers
Java decentralized cross-platform network multiplayer game engine
Cross platform administration tool for Source Engine dedicated servers
The original Xivio.com World Server Core
TR Game Engine allows you to develop your game quickly and easily.
A multiplayer First-Person-Shooter game built in Java 3D
program to manage installation of Minecraft Mods for client and server
java network tictactoe
Multi-user whiteboard emphasizing simplicity.