There is also an open source simulator (called "OpenSim") similar to Linden Labs' Second Life, and this is the basis for a variety of open source "grids". The most popular of the grids would be OSGrid, which works by having server users connect "regions" to the main grid. Another option for the tech-savvy is to connect "standalone" configurations (like the "Diva Distro") through what's known as "hypergrid technology". Almost everything created for Second Life can be used in OpenSim.
Second Life and OpenSim are currently available for Windows, Mac and Linux; both PCs and servers.
Read more about Second Life on Wikipedia!
Read more about OpenSim on Wikipedia!
Learn about Second Life, OSGrid and OpenSim by visiting these sites:
Second Life Official Site
OSGrid - the open metaverse
Metaverse Ink - Standalone But Join the Party