From Wikipedia:

Metaplace provides user-generated virtual worlds. As Metaplace is browser based, worlds made with Metaplace can connect to each other through hyperlinks. Every object in Metaplace has a unique URL. This character of Metaplace allows advanced users to use these URLs in setting up and reading RSS feeds, set up ad services within worlds, and access and show content from the web within their worlds. According to founder Raph Koster, Metaplace seeks to “make online world elements… part of the standard code which drives the web”

Metaplace uses a Lua variant called Metascript. Metaplace users can use Metascript to add functionality to any object in their world. These functionalities can be included in Animals and pets, doors, enemies with the ability to fight and attack, puzzles and games, artsy effects, vehicles.

Metaplace is currently in open beta testing and has created some nifty tools to embed these virtual worlds most anywhere, such as the demo I’ve been playing with below: