OpenSimulator is a 3D Application Server. It can be used to create a virtual environment (or world) which can be accessed through a variety of clients, on multiple protocols. OpenSimulator allows you to develop your environment using the technologies you feel work best - it is designed to be easily extendable through loadable modules to build completely custom configurations. OpenSimulator is released under a BSD License, making it both open source, and commercially friendly to embed in products. Further, any 3D objects and environments you create can be exported and shared with others, similarly you are able to import 3D content and pre-made spaces for use in your own OpenSim environment. You can read more about Open Sim.
An excellent example of an immersive, task based language learning activity in a virtual world can be explored via the 'Chinese island' developed by Scott Grant at Monash University. A range of shared resources including the entire simulation is available for download from the resources section of the site. Read more about the Virtually enhanced languages project at Monash.
A short introduction to the VEL project:
Video by Scott Grant, Monash University.