Node-eventemitter is a project mainly written in JavaScript, it's free.
An up to date (as of node 0.3.2) of the EventEmitter in nodejs
This is an extraction of the EventEmitter functionality from nodejs.
The EventEmitter API is nice, and it would be a waste of effort to implement it on the browser. So, basically this is the code extracted so that it will run on either the client or server side.
See nodejs/Events
See nodejs/LICENSE