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