Fully async, multithreaded, predictive, open source, 3.01 reach
Arianne is an engine to develop multiplayer online games like Stendhal
Full featured MUD server with all the trimmings.
Java Settlers
A Twitch Chat Client
Java decentralized cross-platform network multiplayer game engine
Source for the Flash-based Glitch client
The original Xivio.com World Server Core
Cross platform administration tool for Source Engine dedicated servers
Battleship through network
Modular Minecraft server plugin - commands, events, ...
A multiplayer First-Person-Shooter game built in Java 3D
program to manage installation of Minecraft Mods for client and server