Sorry for late reply... their authoring tools use Unity, not sure about hosting. It's not open source, but it is free (not sure how they plan on making money).
Basically you can create a "world" or a character model and use it or publish it freely, and in-game you can visit other people's worlds.
Basically you can create a "world" or a character model and use it or publish it freely, and in-game you can visit other people's worlds.