For example I have a HTTP-server in node that accepts POST requests and sends their bodies over to an XMPP server.
For a bigger project http://blogspam.net/ aims to classify blog-comments as spam/ham in real-time, and the entire server is written with node.js. (It really just receives JSON objects, scans them with a series of plugins, and sends back a reply saying "OK" or not. Conceptually simple.)
Finally my email stack is powered by node.js, handling 30ish virtual domains with recipient testing, and anti-spam at SMTP-time. This is built around Haraka, which is a reimplementation of the ideas found in qpsmtpd. (A server I once built a small company around.)
Generally I've chosen node because it was fast to get "a scalable server" up and running with. Both the blogspam service and the mail handler were initially perl, and both received 20-40% speedups for free (well if you don't factor in my developer time).