Second Life

Second Life has several up to date Linux clients. It's still a user created world. Opensim is more like the 'wild west,' being run by anyone who wants to set up a server.

One can find some really great places to visit in both Second Life and Opensim. Being user created worlds, the more users over longer periods of time results in higher quality products. Second Life has a huge amount of content for free and for sale on the Marketplace and in-world.

There is literally nothing like it anywhere else.
- Dave