[bes]

Зачем вообще кнопка

<form method="post"> 
	<input type="text" id="tags" style=" width:300px"/> 
	<a href="#"> Selected Tags</a>
	<div id="theWin" style=" background:#f4f4f4; border:1px solid #ccc; margin-left: 300px; padding: 4px; visibility:hidden; position:absolute;">
		<input type="checkbox" name="wordpress" />wordpress<br>
		<input type="checkbox" name="ucoz" />ucoz<br>
		<input type="checkbox" name="dle" />dle<br>
	</div>
</form>

<script>
window.onload = function () {
	var form = document.forms[0];
	var inp = form.children[0];
	var  st = form.children[2].style;
	
	form.onclick = function (e) {
		e = e || event;
		var target = e.target || e.srcElement;
		if (target.tagName == 'A') {
			(st.visibility != 'visible') ? st.visibility = 'visible': st.visibility = 'hidden';
		} else 
		if (target.checked) {
			inp.value += target.nextSibling.nodeValue + ', ';
		} else {
			inp.value = inp.value.replace(target.nextSibling.nodeValue + ', ', '');
		}
	}
}
</script>