Based on simulated 3D environment, Second Life, now can be connected from Linux machines. Linden Lab, the creator of the game has launched public test of the Linux client which supports the same feature set and interface as the Windows and Mac OS X versions.

To start with it first you have to have an account with secondlife.com. It comes to you for free the only catch is here is that you can not buy a land on this virtual world.
The Linux client application is available as a tarball.
After uncompressing the tarball, cd to the directory it creates and run ./secondlife. The in-game interface is built entirely with OpenGL, so you will need a card that supports OpenGL -- but Second Life doesn't depend on any secondary toolkits. Linden Lab doesn't specify system requirements, but the Linux client support forum (which is visible only to registered users) indicates that Linux users are successfully running the app on a wide variety of distributions and configurations.The are more than 140,000 virtual residents and any time at least 3,000 to 5,000 people remain online Start creating objects as soon as you explored the world.