HTML-Code: 
<script type="text/javascript">
function show(id)
{
 if (document.getElementById)
	{
		vis = (document.getElementById(id).style.display == 'block') ? 'none' : 'block';
		document.getElementById(id).style.display = vis;


	}

}
</script>

<a href='#' onClick='show("spoil"); return false;'>Spoiler!</a>
<div id='spoil' style='display: none;'>Blabla</div>