Arianne is an engine to develop multiplayer online games like Stendhal
A simple cross-platform game engine for creating old-school 2D games
CSG for jMonkey
Source code of the chapters of the book 3D Game Development with LWJGL
Java decentralized cross-platform network multiplayer game engine
Java Gamepad API for multiple platforms
AMOS BASIC reimplementation in Java
Modular Game Creation Framework.
The next generation mud is an open-source extensible, programmable mud