Pusher Allows Your Users to Collaborate in Realtime
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.
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.
License: License Free