I touched upon this subject yesterday: there are currently no universally accepted protocols or (de facto or de jure) standards for virtual worlds, like HTTP is on the internet. This lack of standardization means no interoperability between virtual worlds, no exchange of data, avatars or objects between them. When you're in one virtual world, you can't teleport to another!
Of course, several companies and individuals try to fill that gap. There's much at stake: those who create the set of protocols that will eventually rule the metaverse (so to speak), stand to gain a lot of influence - and a lot of money as well.
There are quite a few contenders. I have named a few in the past: Multiverse, a proprietary platform, launched in early August. Yesterday, Metaplace announced they are ready for alpha testing. Metaplace is created by Areae, a company, but is completely built on open standards, to make it open and extendible.
In the mean time, Linden Labs is also contemplating the future. The current Second Life grid cannot scale to the numbers of avatars that Linden Labs would like to see, in a couple of years; a new architecture is necessary. Linden Labs proposed such a new architecture at The First Architecture Working Group Meeting. Linden Labs wants this redesign process to be as open as possible, given the circumstances; you can read about it at the Architecture Working Group wiki. We're all welcome to participate. Linden Labs' current proposal explicitly allows thirdparty hosted sims to be connected to the SL universe, be it as standalone sims or tied together in the grid. Of course, this is all quite preliminary.
Some people just can't wait; they are building their own simulator software, as you can see at opensimulator.org. There are a couple of public grids available to opensim users, for instance osgrid.org or Ruth. A description on how to connect to Ruth can be found on Vint Falken's blog. I like the fact that I can use the default Second Life client to connect to OpenSIM worlds!
I think I'll be watching these developments closely in the coming months. Important things may happen!