<A 
onclick=checkopen(this); 
href="//www.jb51.net" 
target=_blank>打开下载页面1</A> 
<script> 
<!-- 
var original_url = "" 
var adurl = new Array(9); 
var isclick = false; 
var adurl_i = 5; 
adurl[0] = "//www.jb51.net"; 
adurl[1] = "http://www.yuanma51.com/"; 
adurl[2] = "http://www.baidu.com/"; 


function selectFrom(iFirstValue,iLastValue){ 
var iChoces = Math.abs(iLastValue - iFirstValue) + 1; 
return Math.floor(Math.random() * iChoces + iFirstValue); 
    } 

adurl_i = selectFrom(0,2); 

function checkopen(a){ 
if(adurl_i<9){ 
if(isclick==false){ 
original_url = a.href;     
a.href = adurl[adurl_i]; 
     isclick = true; 
     top.location.href = original_url;     
     }else{ 
     a.href = original_url; 
    } 
} 
} 
//--> 
</script>