Catching title, no? Ever heard of LambdaMOO? It's a MUD if the type MOO, ie. text-based multi-user dungeon in which participants may program their own interactive objects and areas. A role-playing game has been incorporated into the world, as well as probably hundreds of personal spaces. Without ever having taken the time to explore Second Life (as it has the unappealing property of requiring you to install something that I wouldn't need otherwise), I do get the impression that it offers much of the same things as LambdaMOO - allowing people to script areas and objects, with the most noticeable difference being that it is rendered in 3D graphics. I am interested in a two-dimensional game with a top-down perspective and of a similar genre as Second Life and LambdaMOO, only better integrated with web, in part in that you may play it without anything else but a modern web browser. People may expand the universe in a similar fashion as making websites - as players are allowed to move their character from a world on one server to the world on another. I imagine that this could be done by some form of automatic user authentication with a server that holds character information. I believe that the Scalable Vector Graphics format to be appropriate, perhaps animated using ECMAScript. While SVG is yet to be natively supported in Internet Explorer, it is relatively well-supported by all the major browsers of today that isn't FAIL. An emerging protocol for bi-directinal communications with web browsers that may also be interesting is Web Sockets. Web Sockets is likely to become a better alternative to Comet &#106avascript technologies, but the last time I checked it was only supported by Mozilla Firefox and Chromium-based browsers.

This isn't reeeeaaaaly what you're asking for but Mine Craft is pretty awesome. Its basically a sandbox game where you build with 3d cubes online. Its free atm but if you pay you get indev access to an rpg survival mode and 50% off the final price.

Infinite maps are being implemented soon and there had been talk of turning it into a quasi-mmo (actually realistic with 200+man servers already up).

Its scripted in Java so can be used ina browser and my ancient laptop gets 300+ fps on fullsized maps with fog off.