I agree with Google Maps Mania on this — I’m quite disappointed that it’s not built on top of Google Earth. However, it’s certainly a unique creation. Google has just launched Lively.
It’s essentially Second Life, but with a million different worlds. You create a “room”, then embed it on your site. People can hop into your room and play around. It’s certainly an interesting idea.
Below is a video with a bit more info, and here are a list of the most popular rooms so far, or go check out GEH Island.