Flash would be a good basis for the kind of thing I'm thinking of. Adobe's already solved a lot of the sandbox issues that would arise, and the language runs faster than JS. In theory, you get each flash player to open 4 or 5 netconnections to other players, get a running pool propagating and distribute the connection list so no one central server is taking the strain of hooking up the peers, you can have filesharing or even distributed processing in the browser. Processing is more interesting to me because it hasn't really been cracked yet. The security overhead to date has been too high. But that, or a java program, or even something written in Unity, could do some amazing stuff with virtually no back-end overhead.

Keep the good work going. I'm interested to see how this will evolve.