FYI, TinyMCE is a nice little editor that makes it super easy to turn a regular textbox into a WYSIWYG HTML editor.

You can play with a demo and see how to install it.

I haven't looked into how to extend it, but if you have a number of textboxes containing editable HTML, you owe it to yourself & your users to bolt something on like TinyMCE.

FWIW I've also worked extensively with FreeTextBox, and while it's nice, installation is a bit more involved, and progress on it seems to have slowed. Plus it's not totally free open source, and the HTML output isn't very XHTML (unless you buy the non-free version).

TinyMCE is cool because it's a literal bolt-on that you can include w/o touching your ASP/ASP.NET/PHP code if you don't want to.

To be fair, another really popular WYGIWYG editor is FCKeditor. I haven't used it, but I've heard it's the bees knees.

And lastly, there's a new potentially-hip WYSIWYM (What You See Is What You Mean) editor cll WYMEditor that I mentioned a while back. Their goal is to keep the outputted XHTML simple, since oftentimes users can paste or SHIFT-click their way to producing crappy HTML that "looks" good to the eye but is horrible (e.g. extra breaks, inline styles) for CMS systems.