Node.js: uses and ideas?

You might wanna check out nodepal - A Drupal integration layer for Node.js developers

This is a module that allows the integration of a Drupal installation with a custom Node.js app. It provides an API so that Node.js developers can directly read and write in Drupal's repository, using constracts like node, user, permission, etc., with having to worry about the underlying implementation and setup of the Drupal installation.

Use Cases:

• You have a working Drupal installation, with users, content, etc. and would like to add real-time services like chat, etc. The usual Apache-PHP-MySQL setup might not the best suited for this kind of apps, so you can develop you app in Node.js and hook it up to your Drupal, using the Nodelpal integration layer.
• You want to build a new Node.js app, that has some notion of users, roles, permissions or content and would like to use Drupal's user, roles, permissions, content model and its great administrative interface, without having to build everything from scratch.
• You want to build a new Node.js app and combine it with Drupal because there is some Drupal module that you find useful. E.g. you can add a Drupal forum, a Blog, or expose some content as a Web Service, or search it with Apache Solr. There is a Drupal module for everything :)

As far as I know, LimitedList (http://limitedlist.com/) is using Drupal 6 + Nodepal + Node.js in production.

There is now also the possibility for nodejs to make query to solr. It can be edit the solr index in real time instead of waiting for the cron to run. Or also to return results without php.