If your are a web developer and you are developing Content Management Systems probably you know what FCKEditor is. If you don't, let me tell you - it is a WYSWIG editor based on JavaScript and it looks and works like MS Word (or similar) and it is available for free. The problem with FCKEditor is that implementation may cause some headaches (specially with .net) and also it is quite heavy. The reason why I chose FCKEditor is built-in image up loader with preview option.
Same guys that developed FCKEditor have released CKEditor - a lightweight and much modern version of FCKEditor. The best thing about that new editor is implementation. It takes seconds and it is fully based on JavaScript:
<script type="text/javascript" src="ckeditor.js"></script>
<script type="text/javascript">
window.onload = function() {
CKEDITOR.replace( 'textareaId' );
};
</script>
For asp.net integration I would use <%= controlId.ClientID %> just in case if ID on output is different.
The downside is that CKEeditor suffers lack of image up loader. That functionality can be found in CKFinder but it is not available for free (price starts form $59).