EverythingServerUbuntu - Ryzom - Ryzom Core Development Site. I should take a closer look at this - at the moment I'm playing a bit of Ryzom, just for relaxation on the weekend, and that has been open source for some time now - you can set up your own server with your own little world. It's not connected to the main system as far as I can see (which is a shame, it would be funny to jump with an avatar from there into my own little world), but still maybe something to play around with. Even if it's probably more complex than OpenSim to build something like Lilliput inside it.