The Multi-Simulator Interface (MSI) is a simulation interconnection engine. It connects simulations by synchronizing their clocks and data. It is a light-weight HLA alternative.
XMUD is a MMORPG implementation in Java. Players can extend the system by creating new objects. The game provides a 3D engine written in Java3D, Persistent state management, synchronization and networking subsystems. Compatible with NWN modules.