Drupal and Node.js
Drupal is a powerful, flexible platform for building web applications, but it's built on the LAMP(ish) stack which has never handled realtime communication easily. Writing an application that could handle massive concurrency has always been hard. Enter Node.js.
Node.js is a breath of fresh air in the Open Source web server landscape. It makes writing applications that handle thousands of open connections at the same time easy.
The Nodejs module integrates Drupal with Node.js, allowing for the best of both worlds. We keep the smarts in Drupal without having to rewrite our business logic while Node.js provides the plumbing to connect a large number of web socket (or other realtime fallback) connections to provide push notification to massive numbers of connected clients in real time.