When would I use JQuery.Callbacks?
$.callbacks.add("foo", myFunction);

$.callbacks().fire("foo", args);

.fireWith(context, args)

var clickCallbacks = $.Callbacks();

clickCallbacks.add(function() { //one one function piece
    //parse and do something on the scope of `this`
    var c = parseInt(this.text(), 10);
    this.text(c + 1);
});
clickCallbacks.add(function(id) { //add a second non-related function piece
    //do something with the arguments that were passed
    $('span', '#last').text(id);
});

$('.click').click(function() {
    var $ele = $(this).next('div').find('[id^="clickCount"]');
    clickCallbacks.fireWith($ele, [this.id]); //do two separate but related things.
});

readyList = jQuery.Callbacks( "once memory" );

ready: function( fn ) {
    // Attach the listeners
    jQuery.bindReady();

    // Add the callback
    readyList.add( fn );

    return this;
}

readyList.fireWith( document, [ jQuery ] );