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.
VR Juggler is a virtual platform for virtual reality application development created at the Virtual Reality Applications Center at Iowa State University. Applications developed in VR Juggler can transparently move between a wide range of VR systems.