If you've read any Neil Stephenson, you may be familiar with Snow Crash. If so, then you will recognize that Croquet is the "Metaverse" in its infancy.
Another explanation is imagine a real-time 3D World Wide Web, where as you create something on your "site" or "world" it is simultaneously present to anyone who is there to see it.
You can even have multiple people pick up and modify the same object at the same time.
Watch this google video to see a presentation of it in action and listen to the developers discuss it.
Wow, I find this very exciting.
The language they're using for the scripting is Smalltalk. Now, I certainly don't think that Smalltalk is the "future of computing", but it goes to show that rarely is the scripting language of choice a critical decision - for nearly any platform. What is revolutionary about this application is the collaborative decentralized peer-to-peer networking subsystem. That is exciting.
EDIT: I just finished watching the whole video (1 hr 14 min). It's good. There's a great demo of what they have so far at the beginning. Midway through, one of the speakers is rather strident about things related to the state of Computer Science at the university level, which is interesting but somewhat unrelated. However, if you wade through that more is discussed about the architecture. There are some fascinating ideas going on. I'd love to see the graphical animation system utilize procedural techniques similar to the upcoming game, Spore.
Here's another Croquet Video. It's actually, a rather better demo.