Very simple JavaScript events system.

Events.js

var Events = {
        /**
         * Given a object will mix the ability to emit events into the object
         * and listen for those events from other functions. Gives the destObject
         * the following three additional methods: on, off, emit.
         * 
         * @param  {Object} destObject
         */
        mixin: function(destObject) {
            var methods = {
                on: function(event, callback) {
                    this._listeners = this._listeners || {};
                    this._listeners[event] = this._listeners[event] || [];
                    this._listeners[event].push(callback);
                },
                off: function(event, callback) {
                    this._listeners = this._listeners || {};
                    if (event in this._listeners === false) return;
                    this._listeners[event].splice(this._listeners[event].indexOf(callback), 1);
                },
                emit: function(event) {
                    this._listeners = this._listeners || {};
                    if (event in this._listeners === false)  return;
                    for (var i = 0; i < this._listeners[event].length; i++) {
                        this._listeners[event][i].apply(this, Array.prototype.slice.call(arguments, 1));
                    }
                }
            };

            for(var methodName in methods) {
                if (methods.hasOwnProperty(methodName)) {
                    destObject.prototype[methodName]  = methods[methodName];
                }
            }
        },

        /**
         * Creates a object with the ability to emit events and
         * be listened too.
         * 
         * @return {Object}
         */
        create: function() {
            var newObj = {};
            this.mixin(newObj);
            return newObj;
        }
    };

usage.js

// centralised notification center
var notificationCenter = Events.create();

notificationCenter.on('message', function(message) {
    console.log('MESSAGE RECIEVED: ' + message);
});

notificationCenter.emit('message', 'hello world');

// mixing events into an existing object
var app = {
    boot: function() {
        // do somthing
        this.emit('ready');
    }
};
Events.mixin(app);

app.on('ready', function() {
    console.log('App booted and ready!');
});

app.boot();