Sirikata is an BSD licensed open source platform for virtual worlds. We aim to provide a set of libraries and protocols which can be used to deploy a virtual world, as well as fully featured sample implementations of services for hosting and deploying these worlds. We are aiming for an alpha release in late Q1 2009 and this video teaser should give you some sense of what to expect.
Website at sirikata.com
More videos: Sirikata
Song Credits: http://ccmixter.org/files/duckett/17456