realXtend offers a free open source virtual world platform with which you can create your own applications using it as a base.

The realXtend viewer is an open source browser (GPL license) that you can use to experience rich multiuser virtual worlds.

The realXtend server is an open source server (BSD license) with which you can make your own virtual worlds available to a larger audience. The server has a sample virtual world that you can utilise as it is or you can modify it to suit your needs. realXtend also offers free, high-quality content for your own worlds.

realXtend mission is to speed up the development of the global standardized 3D-web of virtual worlds by making the best technology available to everyone, and entirely free of charge.

http://www.realxtend.org/ (less info)