Although I can't actually say anything about it, as the Alpha program's terms and conditions have strict non-disclosure clauses, Metaplace is multi-world, 'play' in browser virtual worlds environment for which users can create their own worlds.
One of the things I really like about this idea is that you can link between worlds. Anyway - without reproducing their press-blurb, that is probably all I can really say. Worth taking a look, in my opinion, but I don't know how well it will scale to multiple users, or deal with different in world physics simulations yet (most of the "worlds" I have looked at have been 2D games)
Keywords: connectivity, metaplace, virtual worlds