Pusher is a realtime service that complements your existing server architecture, which allows your users to collaborate in realtime. It Adds live status updates and messages. And it lets you broadcast events to everyone viewing a page.

When there is an activity that you want several users to know about, your server makes a simple REST call to the Pusher API passing a JSON payload of data. This message is modelled on Javascript event triggering. The event is relayed to the relevant browsers on the specified channel. Javascript code bound to the specified event informs the connected users of the activity.

However, they are currently limiting signups to Pusher. This is so that they can monitor usage of the system and make some tweaks without breaking things for everyone.

Requirements: -
Demo: http://pusherapp.com/demos
License: License Free