//Get required href
sHref = "http://zoocha.com";

//Open window and then blur
window.open(sHref,"_blank","toolbar=1,location=1,directories=1,scrollbars=1,resizable=1,status=1,menubar=1").blur();

//Forces current window to stay on top
window.focus();

http://daipratt.co.uk

Launch a popunder using Javascript that has a default browser setup

JavaScript