There are a number of virtual worlds that are lively places to visit, generally focused on entertainment. These are Second Life®, and a number of worlds (to many to list here) based on the OpenSimulator (OpenSim) source code, providing some limited of compatibility with the Second Life® world. From this project, several branches have been taken for as many diverging purposes as was required for them. It may all depend on what will work best for what you want to accomplish or to do for which simulator option to use.
All the world simulators you can install yourself, come with limited content. While you can create a world and lands in it, it is really a blank sandbox waiting for your designs to be put in it and who will be interested in sharing it with you!
While Discover Creo Mundos is open for many virtual world platform development concepts and promotion, several projects are only for the support and use of the Halcyon Simulator and the web worlds as far as is possible.
The Halcyon projects include:
The multi-world Marketplace which may include the web world technologies. Website is currently in design stage.
My World website front end for Halcyon worlds, providing world management and user access to your world. This project also provides the website for Discover Creo Mundos.