Minecraft does not provide a way to see who's currently online on a server at any given time, unless you log in. The craftinfo script parses the minecraft server log incrementally and provides the result using a gevent powered echoserver.
Connections to this socket server can be made from different clients, like a web application, a little desktop app or a mobile app.
Anywho, for the time being this script only serves a personal need. You can use it, but you might not find it usable.