MudNode is a barebones node.js implementation intended to run on heroku using heroku's new persistent cedar stack. By using heroku you get 100% free hosting for your game on industry standard, managed hardware. You get unlimited harddrive and bandwidth for the project, and 240 megabytes in a managed mongodb that's located near the EC2 instance (also in EC2). If you ever get to the point where 240 megabytes isn't enough it's fairly cheap to upgrade. Or you can run on your own/dedicated server.
Heroku isn't required. This will run anywhere that node.js is installed. It uses socket.io and a web client by default.
The github page includes instructions for getting it live on heroku.
Concurrency is a feature
The code includes database, web server, and socket examples.