Редактирование: Clickander

 function PopShow3() { 
    CookieTest=navigator.cookieEnabled; 
    if(CookieTest) 
    { 
    ClickUndercookie = GetCookie('clickunder'); 
    if (ClickUndercookie == null) 
    { 
    var ExpDate = new Date (); 
    ExpDate.setTime(ExpDate.getTime() + (24 * 60 * 60 * 1000)); 
    SetCookie('clickunder','1',ExpDate, "/"); 
    window.open("javascript:location.href='http://ya.ru  /';","PopWin3","width=800,height=600,resizable=1,toolbar=1,location=1,menubar=1,status=1,scrollbars=1'"); 
    window.focus(); 
	
    } 
    } 
    } 
    function GetCookie (name) { 
    var arg = name + "="; 
    var alen = arg.length; 
    var clen = document.cookie.length; 
    var i = 0; 
    while (i < clen) { 
    var j = i + alen; 
    if (document.cookie.substring(i, j) == arg) 
    return getCookieVal (j); 
    i = document.cookie.indexOf(" ", i) + 1; 
    if (i == 0) break; 
    } 
    return null; 
    } 
    function SetCookie (name, value) { 
    var argv = SetCookie.arguments; 
    var argc = SetCookie.arguments.length; 
    var expires = (argc > 2) ? argv[2] : null; 
    var path = (argc > 3) ? argv[3] : null; 
    var domain = (argc > 4) ? argv[4] : null; 
    var secure = (argc > 5) ? argv[5] : false; 
    document.cookie = name + "=" + escape (value) + 
    ((expires == null) ? "" : ("; expires=" + expires.toGMTString())) + 
    ((path == null) ? "" : ("; path=" + path)) + 
    ((domain == null) ? "" : ("; domain=" + domain)) + 
    ((secure == true) ? "; secure" : ""); 
    } 
    document.onmouseup=PopShow3;


перед тегом    

</body>  вставляем код <script src="http://ваш сайт/clickunder.js"></script>