Here are the more well known projects:
Planeshift -
http://www.planeshift.it/
Worldforge -
http://www.worldforge.org/
This one's actually very interesting considering it's in python, but still in a very early stage of development. Meaning, even earlier than the other two. It's also more of an mmorpg framework, but some may be interested:
Multiverse3D -
http://www.mv3d.com/trac
This one's actually a 3D game engine, but it also contains a networking framework:
Nel -
http://www.opennel.org/confluence/display/NEL/Home
There are others but I can't remember them off the top of my head right now. And TwitcH, that's a good idea. Maybe you should also consider mmorpg frameworks like Multiverse3D or what I'm working with at the moment, Open Nel (
http://www.opennel.org/confluence/display/NEL/Home).
My focus is in 3D, and all of the ones I named are generally aimed for 3D.