I have created a simple I-Room from scratch, built that into a complex Unity3D terrain and incorporated very simple avatars as an exercise in learning about Unity3D.  I have also experimented with two different multi-user server addons - SmartFox Pro and Photon. I have embedded the Unity3D virtual world view into a web page alongside collaboration widgets to provide Twitter following of a given #tag, VoIP via Tokbox, and a Dabbleboard for showing presentations and giving a shared whiteboard. The demo systems are all available via http://www.aiai.ed.ac.uk/~ai/unity/

Even better, there are a number of groups interested in content exchange from OpenSim to Unity3D. An example is the recent conversion of the OpenVCE collaboration region which was originally on the Second Life VCE region, ported to OpenSim and then converted through Tipodean's Collada Converter for use in Unity3D. It is now available for use under the flexible Lesser GPL licence.