OpenSimulator

An alpha-level open-source version of Second Life, compatible with the old Second Life Viewer and similar programs but not under the control of Linden Lab. Work is in progress to allow avatars to teleport between Second Life and the many OpenSim grids, but everything is still experimental.

Want your own corner of The Metaverse but don't want to pay Linden Lab for server space? Don't mind a few bugs and a slightly reduced feature set? Know how to compile code and use a command line console? Then run a sim of your own on your own computer! There are lots of public grids you can hook your sim up to, or you can run an entire grid of your own.

Get it here: The OpenSimulator Project