If you have been perusing my 3D Web page, you know that Opensimulator is a piece of technology that I have been investing time in and keeping my eye on. I have links to some rudimentary instructions for getting online/in-world, but there is also a good introduction on the Hypergrid Business blog.
Hypergrid Business is a primary source for seeing what business and education are doing in OpenSim.
Check out this article: What is OpenSim?