I have had good luck using jQuery and the jHTMLArea WYSIWYG editor nice and simple.
All you have to do to implement it is:
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js"></script>
<script type="text/javascript" src="scripts/jHtmlArea-0.6.0.min.js"></script>
<link rel="Stylesheet" type="text/css" href="style/jHtmlArea.css" />
<textarea id="htmlEditor" cols="50" rows="15">This is some text for editor</textarea>
<script type="text/javascript">
$(function() {
$("htmlEditor").htmlarea();
});
</script>