Open Sim is an open source virtual world compatible with the second life viewer. Very cool.
Here's a video showing S4SL being used with Open Sim:
I've gotten messages from several people interested in using S4SL on Open Sim. Feel free to share what you are doing on the comments here, so people can learn from each other!
UPDATE 7/10/09: There is now a universal scratch 4 opensim package here!