A script that shows a popup in the middle of the screen.

popup.js

function popup(p_url, p_title, p_width, p_height) {
	var left = (screen.width * .5) - (p_width * .5);
	var top = (screen.height * .5) - (p_height * .5);
	var target = window.open(p_url, p_title, 'toolbar=no, location=no, directories=no, status=no, menubar=no, scrollbars=yes, resizable=no, copyhistory=no, width=' + p_width + ', height=' + p_height + ', top=' + top + ', left=' + left);
}