<html>
<head>
<title></title>
</head>
<body>
<div id="code">
<p>aaaaaaaaa<br />
<p>bbbbbbbbb<br />
<p>ccccccccc<br />
<p>ddddddddd
</div>
<script type="text/javascript">
lis=document.getElementById('code')
for (var i=0;i<lis.getElementsByTagName("p").length;i++){
x=i+1
firsti = lis.getElementsByTagName('p')[i]
newListElem = document.createElement('span')
newListElem.innerHTML = x+' | '
lis.insertBefore(newListElem, firsti)}
</script>
</body>
</html>
так?