레이어 팝업 중앙 띄우기

var left = ( document.body.scrollLeft + (document.body.clientWidth - $(".popup_div").width()) / 2 );
var top = ( document.body.scrollTop + (document.body.clientHeight - $(".popup_div").height()) / 2 );

---

var $layerPopupObj = $('.layerPopupBox');
var left = ( $(window).scrollLeft() + ($(window).width() - $layerPopupObj.width()) / 2 );
var top = ( $(window).scrollTop() + ($(window).height() - $layerPopupObj.height()) / 2 );
$layerPopupObj.css({'left':left,'top':top, 'position':'absolute'});
$('body').css('position','relative').append($layerPopupObj);

출처 http://mytory.net/archives/812/