gistfile1.rhtml

<div id="preview"></div>

<%= javascript_include_tag ["showdown"] %>
<script>
document.observe("dom:loaded", function() {
	var converter = new Showdown.converter();

	$("post_content").observe("keyup", function(e) {
		$("preview").innerHTML = converter.makeHtml($("post_content").value);
	});
});
</script>