Metaplace.com more than just a virtual world.  You can build a world on Metaplace.com and you can build a world on your web page or blog, but best of all its drag and drop, you can build your own games etc etc.  Still in beta and so you have to apply to get access to it which can take weeks ... from what I hear it is well worth it.

 And here is a explaination of how it works from the founder Ralph ...