CoffeeMud is a text-based virtual reality game engine (a MUD). It is a mature, full-featured Java codebase. It includes web (HTTP) and email (SMTP) servers, chat (IMC2 and I3) and web clients, and supports ANSI, MXP, and MSP.
Exipnos is an Open Source, network based 3D virtual reality platform. The platform is intended mainly (but not exclusively) for eLearning applications. Role Player or Adventure Games might as well be developed and hosted on Exipnos.