跳转到当前页：
<select name="here " onchange="location.href=this.options[this.selectedIndex].value;">
<option value= "http://www.163.com "> 163 </option>
<option value= "http://www.sina.com "> sina </option>
<option value= "http://www.tom.com "> tom </option>
</select>
跳转到新页面：
<select onchange="window.open(this.options[this.selectedIndex].value)" name="select">
<option value="http://www.163.com" selected="selected">163 </option>
<option value="http://www.sina.com">sina </option>
<option value="http://www.tom.com">tom </option>
</select>