I've written several tools which need to send data to "a server" of some kind. Writing trivial node.js servers is painless, and the event-based approach makes them scale really well.

For these ad-hoc kind of systems it is hard to think of anything as fast and simple to get started with. (Sure you could use Twisted, Sinatra, or some other framework, but node.js is very very simple to get started with which helps a lot.)