Desktop/Android/HTML5/iOS Java game development framework
Arianne is an engine to develop multiplayer online games like Stendhal
Java Settlers
8-bit Graphical API for Java
An environment to develop games based on a rectangular board
Map editor for the game Can I Scape
Toolkit for the easy prototyping of hybrid games for Tabletop Devices
Java Gamepad API for multiple platforms
A multiplatform game engine for interactive fiction experiences
An extensible development framework for roleplay games.
Open Source Virtual World and MMOG Development Platform
A tools API for designing table top game utilities
The next generation mud is an open-source extensible, programmable mud