Exactly what you wanted: http://realitymachine.net/asteroids.html

And the HN article, if you want to drum up some more players: http://news.ycombinator.com/item?id=1738543

I hacked together a very (very) basic multiplayer with a paper-thin node.js server in coffeescript.

Yes, really: http://github.com/cushman/asteroids.js

It might deserve a newspost of its own, but I'd like to have a working server running so people can actually, you know, use it. Unfortunately, I haven't thought to get node running on my VPS before now, and it's almost 02:00 here...