function setag(){
 var tagg=window.showModalDialog("tags_main.php","tag","dialogWidth=800px;dialogHeight=600px"); 
  if(typeof(tagg) != 'undefined') document.form1.tags.value=tagg; 
      }

//选择关键字
var tag="";
function selectTag(str)
{
tag=tag+","+str;
if (tag.substr(0,1)==',') tag=tag.substr(1);
 $("#selecttag").val(tag); 
}

function selectTagOK()
{
window.returnValue= $("#selecttag").val(); 
window.close(); 
}

<div style="padding-left:20px;border:1px">所选TAG: <input type='text' id='selecttag' name='selecttag' size='80'/><input type="button" onClick="selectTagOK()" value="确定选择"/> </div>

<a href="../tags.php?/<?php echo urlencode($fields['tag']); ?>/" target="_blank">{dede:field.tag /}</a>

<a href="../tags.php?/<?php echo urlencode($fields['tag']); ?>/" target="_blank">{dede:field.tag /}</a> 
<a href="#" onclick="selectTag('{dede:field.tag /}')">[选择]</a>