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function selLink(){
    selurl=document.myform.mysel.options[document.myform.mysel.selectedIndex].value;
    if (selurl != "" )location.href=selurl;
}

<form name="myform">
<select name="mysel" onChange="selLink()">
<option value="">選択したページへ</option>
<option value="http://www.nicovideo.jp/">ニコニコ動画</option>
<option value="http://www.youtube.com/">YouTube</option>
<option value="http://www.yahoo.co.jp/">Yahoo</option>
</select>
</form>