jQuery.fn.center = function () {
    this.css("position","absolute");
    this.css("top", ( $(window).height() - this.height() ) / 2+$(window).scrollTop() + "px");
    this.css("left", ( $(window).width() - this.width() ) / 2+$(window).scrollLeft() + "px");
    return this;
}

$(document).ready(function(){
 $('#wrapper').center(); 
 $(window).bind('resize', function() {
        $('#wrapper').center({transition:300});
    });
});