Kitely have launched their Virtual Worlds on demand service. No, this isn't an April Fool's joke! What you will need to take advantage of their free beta, is a facebook account and an SL client. And, to take advantage of their free credits offer, 20 friends. Once you get to their website, you can sign in using your Facebook account, and then choose to visit their sample sim, or create your own world.
I found it a little confusing at the very beginning, finding myself in the sample sim, when I had expected to be in my own sim, but I soon worked out that one needs to go to the "my worlds" tab to create your own world... if you simply click to enter world once you have logged in, you will find yourself on a sample sim world with some empty exhibition spaces.
Having made your own world, and logged in using the SL client, you will find yourself on your own version of an SL sim. This only persists while there are avatars online inside it... it shuts down and goes into cold storage once everyone has left.
A word: if avatars come to your world and haven't been anywhere else in Kitely, the creator of the world gets 200 kitely credits. You only have this ability the first time you go into Kitely. My world is here. Kitely advertise free credits if you link Kitely with your facebook... but you only get these if you have a picture on your account and at least 20 friends.
It seems that it isn't currently possible to teleport from world to world, so you have to log out to go anywhere else. Also, it doesn't give you a chance to use the viewer of your choice - it will use the SL viewer unless you choose another as your default. Imprudence should allow importation and export of objects.