A really simple lightbox overlay MooTools Class.

lightBoxOverlay.js

// Lightbox Overlay
var LightBoxOverlay = new Class({
    Implements: [Options, Events],
    options:{
        tag: 'div',
        properties: {
            id: 'lightbox_overlay',
            'class': 'hide'
        }
    },
    initialize: function(el, options){
        this.setOptions(options);
        this.element = $(el) || this.createElement(this.options);
    },
    toElement: function(){
        return this.element;
    },
    createElement: function(options){
        return new Element(this.options.tag, this.options.properties);
    },
    show: function(){
        this.resize();
        this.fireEvent('show');
        return this;
    },
    hide: function(){
        this.fireEvent('hide');
        return this;
    },
    resize: function(el){
        var size = ($(el) || window).getScrollSize();
        this.element.setStyles({
            'height': size.y,
            'width': size.x
        });
        return this;
    }
});