Solipsis is a pure peer-to-peer system for a massively shared virtual world. There are no central servers at all: it only relies on end-users' machines.
The initial Solipsis project has ended some time ago when the core team left the project. The original solipsis web site is available here.
Since then, Joaquin Keller started Twinverse -a virtual world based on geography- and Gwendal Simon took a research and teaching position at Telecom Bretagne.Solipsis.org is an academic project inspired by the original Solipsis.
Pr. Shun-Yun Hu is one of the early researchers in peer-to-peer virtual worlds and maintains a good page listing all the initiatives in this area.