Discord is highly expressive, text-based MMO engine written in JavaScript on top of Node.JS and MooTools.

This is packaged as a library. Just add discord to your package.json file, then type npm install.

Here's an example world using the Discord engine:

require('discord').start({
	world_name: 'DiscordMUD',
	world_path: __dirname+'/world',
	start_room: 'lobby'
}).listen(8000);

For more information, check out http://github.com/Yuffster/discord-example.