Arianne is an engine to develop multiplayer online games like Stendhal
...The engine Marauroa provides a simple way of creating games on a portable and robust server architecture. Marauroa manages the client server communication and provides an object orientated view of the world for game developers. It handles database access in a transparent way to store player accounts, character progress and the state of the world
Open Source Virtual World and MMOG Development Platform
3D Client (Windows), Server, and Tools to easily create massively multiplayer games and virtual worlds.
If you want to contribute to the source code please go to https://github.com/multiversemmo/MultiversePlatform