OpenSim is an open source server implementation of a virtual world quite similar to Second Life but with many differences. It is a lot more extensible and can be run standalone or connected to a grid, being a grid a bunch of servers all connected together so that the land they host can be accessible from each other.
There are a number of grids running OpenSim code, some of them are commercial and some are not. In OSGrid, for instance, you can connect your server to their grid and it will be available straight away. You will need to set up your server properly and it is not an easy task if you don't have the needed skils. For this reason a number of grids are offering this service for you. Depending on how much you want to spend, you can have as much virtual land as you want in grids like Openlife, Avatar Hangout or ReactionGrid, the latter one being a fairly new proposal(at least for me) that seems to be focused on Education and Business.
I discovered ReactionGrid the other night through the Sloode development team as they are working on a port of their fantastic project to OpenSim. I personally cannot wait to test it! Hope to have time to do it very soon. Another cool bit about this grid is that they have some kind of integration with Skype. I haven't had time to test it yet but this is quite interesting for me as we have been working in the same functionality for a while in our current project.
I am currently hosting a region in OSGrid called Galicia. Feel free to call by and say hello!