The recent opening up of the Second Life Viewer code was big news, but the thing that everyone is waiting for is for the server-side stuff. Well, that may be a while off - see my interview with Cory Ondrejka for some more background on this. Meanwhile, though, the libsecondlife group has taken under its wing the OpenSim project, which as the home page puts it laconically:
OpenSim is a project to develop an Open Source Simulator.
This is great news, because it means that people can start developing other Second Life-like virtual worlds, completely independently of Linden Lab. It will also mean that people can start to explore some of the thorny issues of multiple, compatible virtual worlds now.