Thought I saw some discussion of this a while back.  Just a few links:

http://www.opencroquet.org/             which begat
http://www.opencobalt.org/home         and
http://code.google.com/p/openqwaq/    which is the uncle? of
http://virtualworldframework.com/

Nikolay Suslov also had a very nice version which I don't have a link for
right now.
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