Metaplace, a service that lets you design your own virtual worlds, has now launched embedding, so that you can stick your world (or your friends' worlds) on your website.
* You can use it as a gathering space for your community — on forums, for example, or as this user has done, as a way to foster community for her comics shop.
* You can make it the central gathering spot for a Ning community, as 3DSquared has done for the Digital Workforce Intensives in Louisiana.
* If you're a musician, you can hold your live shows right there on your site, complete with streaming audio and an audience, as Grace McDunnough plans to do.
* Over time, crazier uses will come about, such as this experiment in using virtual worlds to annotate the real world by embedding a Metaplace world on a geolocation in Google Maps.
(Disclosure: I am an advisor to Metaplace)
