OpenSim

OpenSim is an open-source virtual world. The server-side software was built from scratch based on the SL viewer so interaction and features are very similar. Still in the alpha stage of development it has reached a point of actually being useful. It lacks the closed, serious business model Linden has developed SL into. Multiple grids have been developed and there is the option of setting up a standalone sim. Standalone sims, grids and sims linked to grids can also connect to Hypergrid which is basically the Internet of virtual worlds.

With OpenSim you can use your own server and not have to pay hundreds of jewgolds for a sim that you are only renting. You can also run as many sims as you want. You may freely choose names and grid locations in most cases and change either one on a whim without filing a support ticket or paying fees. Or just be your own grid.