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.
The VROnline project is universal 3D virtual reality engine for games with client/server architecture, free 3D camera, user configurable actions and AI scripts. It's based on C# and directX (now 9.0) technology. It can manage many data models(rpg,sims).