Here is another interesting link passed on from a friend (thanks, Roger!) - Project Wonderland - Toolkit for building Virtual Worlds. Seems like a cool new java-based toy for the desktop that lets users create interactive 3D virtual worlds like Second Life for themselves. There is no license fees to pay - the platform is available under the GNU GPL v2.0, though commercial customers might want to look into the dual license.
Interesting as the project may sound, questions abound about how useful this is if it is going to be just a few developers creating things on their own servers - even if a few of them manage to link up. The beauty of virtual worlds is the ability to be physically be in one corner of the world while you are interacting virtually with people all over the world. The more people you see around and the more content you let them generate and play with, the more likely users of the world are going to last.
It is the being linked in to other people that allows virtual worlds flourish - not just the virtual part. It will be interesting to see how this effort fares outside of a purely educational experience.