Desktop/Android/HTML5/iOS Java game development framework
A simple cross-platform game engine for creating old-school 2D games
Java Settlers
8-bit Graphical API for Java
Example code for creating a 2D platform game in Java.
Source code of the chapters of the book 3D Game Development with LWJGL
Open Source Virtual World and MMOG Development Platform
AMOS BASIC reimplementation in Java
The next generation mud is an open-source extensible, programmable mud