LIMBO is a simple collaborative program that allows multiple participants represented as avatars to load and manipulate 3D models in a persistent virtual environment. This program comes complete with source code which you can use as a starting point for developing your own collaborative applications. 

LIMBO bases its networking on CAVERNsoft so be sure to download that also. 

There are currently two flavors of LIMBO: 

Just LIMBO 0.5 (updated 9/14/99) 

and 

LIMBO/VTK 0.22 (updated 2/1/99) : this flavor demonstrates how to use the Visualization Toolkit with LIMBO. 
 

LIMBO Snapshot