Here are a few practical applications of Node.js:
A Successful Web Front-End Refactor
Klout is a company that measures people's influence based on their activity in social media. We make use of their API at Table XI. Klout was powering a web interface to give users access to their scores with a LAMP (Linux/Apache/MySQL/PHP) stack. They found this was not scaling well as more users accessed the application. They rebuilt this interface using Node.js., and it's now handling tens of thousands of concurrent users on two servers.
Read more: "The Tech Behind Klout.com"